TL;DR. Merger arbitrage is a deceptively simple trade: when Company A agrees to buy Company B at a fixed price, B’s stock should rise to the deal price as soon as the announcement hits. It usually does not get all the way there. The gap that remains — the arb spread — is the market’s price for two risks: the deal failing, and the wait until close. Buying the target and (sometimes) shorting the acquirer locks in that spread if the deal closes on time. Lose enough deals, and the spread on the winners is no longer worth the risk.
The setup, in one trade
An acquirer announces it will buy a target at a fixed price — cash, stock, or a mix. Once that price is signed and the contracts are public, the target’s stock stops trading on its own business prospects and starts trading on the probability the deal closes. The arb spread is the gap between today’s target price and the price you will get at close.
The formula is straightforward:
Spread = Deal price − Current target price
Spread % = Spread / Current target price
Annualised return ≈ Spread % × (365 / Days to expected close)
Buy the target at the current price. If the deal closes on schedule at the agreed price, you collect the spread. If the deal breaks, the target typically falls back toward where it traded before the bid — sometimes well below. Either way, time is your tax: the longer the close, the lower your annualised return on the same dollar spread.
A worked example with a live deal
Take Paramount Skydance’s $31.00-per-share all-cash bid for Warner Bros. Discovery. The deal was announced February 27, 2026, shareholders approved on April 23, 2026, and the U.S. Department of Justice cleared the transaction on June 12, 2026. Targeted close is September 2026, with the EU’s decision expected July 7 and the UK CMA’s decision expected August 7.
WBD closed at $26.83 on June 15, 2026 (one trading day after DOJ approval, source: Nasdaq composite close). Plug in the numbers:
| Inputs | Value |
|---|---|
| Deal price (cash) | $31.00 |
| WBD price, June 15, 2026 close | $26.83 |
| Gross spread per share | $4.17 |
| Gross spread % | 15.5% |
| Days to targeted close (Sept 30, 2026) | ~107 |
| Annualised gross return (simple) | ~53% |
A 53% annualised gross return on a deal where DOJ has already cleared looks like free money. It is not. The market is telling you something. With EU and UK reviews still pending and a September target close, the market is pricing material residual risk — the chance that Brussels or London extracts remedies that push the deal out, or that something else breaks before money changes hands. The spread is the rent on that uncertainty.
What drives the spread
Every arb spread reflects four ingredients, blended into one number:
- The deal-break probability. Higher perceived risk = wider spread. Regulatory pushback, financing wobbles, hostile target boards, market dislocations, and material adverse change (MAC) clauses all widen spreads.
- The downside if the deal breaks. If the target trades to $20 on a break versus the $26.83 today, the loss is bigger than if it trades to $25. Arbs pay attention to the unaffected price — where the stock traded the day before the leak or announcement.
- Time to close. A spread that closes in 30 days is more valuable than the same spread that takes 12 months. The annualisation math is unforgiving for long-dated deals.
- Carry costs. The financing cost of holding the long, the borrow cost of any short hedge, plus dividends, plus taxes and trading costs. None are free.
Two-leg trades: stock-for-stock deals
When the acquirer pays with its own shares (a fixed exchange ratio), the arb position becomes two-legged: long the target, short the acquirer in the ratio specified. This locks in the spread regardless of how either stock moves between today and close.
Example. Suppose acquirer X offers 0.5 shares of X for every 1 share of target T. If X trades at $40, the implied value of T is $20. If T trades at $18 today, the spread is $2 per T share, or 11.1%. To capture it, buy 1 T at $18 and short 0.5 X at $40. At close, the long T turns into 0.5 X (covering the short) and you collect the $2 difference in cash — minus the borrow on X and any other carry.
Visualising the spread over the deal’s life
The chart below sketches a typical hard-cash arb’s spread trajectory through the deal calendar: a wide gap right after announcement, gradual tightening as milestones clear, the occasional widening when a regulatory wobble hits, and a sharp convergence at close.
The asymmetric payoff every arb learns to respect
Merger arbitrage looks like a yield product but pays like a short put. Most deals close, returning the small spread. A few deals break, and the loss on a broken deal is several times the size of the spread you captured on a successful one. Academic research has shown that a diversified merger-arb portfolio earns positive returns most months and a chunky negative return in a small minority — a profile classic enough that Mitchell and Pulvino likened it to “writing uncovered index put options on the market.”
The chart below makes that asymmetry concrete using the PSKY-WBD numbers. Buying WBD at $26.83, the maximum gain at close is the deal price of $31.00 — capped, no matter how attractive WBD’s standalone business looks. If the deal breaks and WBD reverts toward its pre-bid trading range near $20, the loss is roughly $6.83 a share — about 1.6x the $4.17 maximum profit. That is the payoff every arbitrageur is implicitly accepting.
That is why professional arbs do three things:
- Diversify across many deals. A handful of broken deals in 50 positions is survivable. The same break rate in 5 positions is not.
- Size by expected loss, not by deal size. The right question is “if this deal breaks, how much do I lose?” not “how much capital fits in this deal?”
- Stay liquid into the close window. Most of the spread is captured in the final weeks. That is also when an unexpected regulator letter can detonate the entire position.
Common mistakes
- Confusing the gross spread with the achievable return. The 15.5% headline shrinks fast once you subtract financing, borrow on the short leg (in stock deals), dividends, transaction costs, and taxes.
- Annualising naively. A 2% spread over 30 days is a 26.8% annualised return only if you can redeploy capital into another 2%-in-30-days deal the moment this one closes. Most can’t.
- Ignoring the unaffected price. The downside on a break is not “the offer price minus today’s price.” It is “today’s price minus where the target trades after the bid is withdrawn.” That can be much lower than people assume.
- Trusting the announced close date. Deals slip. The base rate for at least one extension on a deal involving multi-jurisdiction regulators is high. Budget for it.
- Holding through a hostile counter-bid without re-underwriting. A surprise topping bid is great if you’re long. A failed counter is the kind of week-from-hell that takes a year of returns with it.
What to learn next
Merger arbitrage is one branch of a wider tree called event-driven investing. Adjacent strategies include distressed-debt arbitrage, SPAC arbitrage, spinoff investing, and special-situations equity (tender offers, going-private deals, post-bankruptcy equity). The math — spread, probability, time, downside — is similar. The legal documents, the cash-flow profile, and the failure modes are not. The next concept worth tackling is break-fee analysis: the reverse termination fees built into modern merger agreements, which set the floor on what the buyer pays if it walks away — and which directly compress (or widen) the arb spread on real deals.
Sources
- ECMSource: DOJ clears Paramount Skydance’s $111B Warner Bros deal (June 12, 2026) — verified deal terms used in the worked example.
- Mark Mitchell and Todd Pulvino, “Characteristics of Risk and Return in Risk Arbitrage,” NBER Working Paper No. 7711 — canonical academic treatment of merger-arb returns and the “selling-put-options” payoff profile.
- Federal Reserve H.15 Selected Interest Rates — benchmark for financing-cost assumptions when comparing arb spreads to risk-free returns.
- SEC Investor Bulletin on tender offers — primer on the legal mechanics of cash tender offers that underpin many cash mergers.
- Maureen Farrell and James B. Stewart, Hedge Hogs; and the classic Goldman / Drexel-era literature on risk arbitrage as a distinct discipline.
Disclosure: This article was produced with AI assistance and reviewed before publication. It is for informational purposes only and is not investment advice.