The moment a public company acquisition is announced, a peculiar opportunity appears in the market: the target's share price jumps, but almost never all the way to the deal price. The gap between current price and deal price — the spread — exists because the deal might not close. Merger arbitrageurs are the investors who buy that gap and bet on completion.
How a cash deal works
A company trades at £20 per share. An acquirer announces a £25 all-cash offer. The target's stock immediately jumps to — say — £24.50. The 50-pence gap to the offer price represents the annualised return available to an arbitrageur who buys at £24.50 and receives £25 at close.
If the deal closes in three months, the 50-pence gain on a £24.50 investment represents approximately 2% over three months — roughly 8% annualised. That is the arbitrage return, assuming the deal closes exactly as announced.
Stock-for-stock deals: the two-legged trade
When the acquirer pays in its own shares rather than cash, the deal price floats with the acquirer's stock. If Acquirer A offers 0.8 of its shares for every 1 target share, and Acquirer A trades at £30, the implied offer price is £24 per target share. If the target trades at £23, the arb buys the target at £23 and simultaneously short-sells 0.8 Acquirer A shares at £30 per share.
This hedge locks in the spread regardless of broad market movements — if both stocks decline together, the long and short positions offset each other. The arb profits if the deal closes at the agreed exchange ratio, entirely independent of what the overall market does in the intervening period.
What determines the spread
The spread reflects three things: the market's implied probability of deal completion, the expected time to closing, and the risk-free rate (the opportunity cost of the capital deployed). A wider spread signals greater market scepticism about completion — the market is pricing in meaningful break risk. A narrow spread signals high confidence.
What breaks deals — and why it matters
The primary risk in merger arbitrage is deal break — the acquisition fails to close and the target's stock collapses back toward (or below) its pre-announcement price. The 50-pence spread gain evaporates and is replaced by a loss of several pounds per share. Because arb desks are often leveraged, deal breaks can produce losses that are multiples of the spread being captured.
Deals break for several reasons: regulatory rejection on antitrust grounds (increasingly common in tech and pharma), target shareholder vote failure, invocation of material adverse change (MAC) clauses when the target's business deteriorates, financing failure in leveraged transactions, or the acquirer choosing to walk away and pay a break fee. Regulatory risk has become particularly significant — US, EU, and UK authorities have all become more assertive in recent years, widening spreads on deals with obvious competitive concerns.
The analytical framework
Professional arb desks assess every announced deal across several dimensions before taking a position: the strategic logic (does this acquisition make commercial sense for the buyer?), the regulatory profile (what market concentration does it create, and which authorities must approve?), the financing arrangements (is the debt fully committed and from credible lenders?), and the nature of the transaction (friendly deals backed by target board have meaningfully higher completion rates than hostile bids). The return calculation is mechanical — the probability assessment is where the edge lies, and it requires genuine sector knowledge, regulatory expertise, and legal analysis rather than quantitative modelling alone.