The thermolysis of homoallyl ethers proceeds in a concerted manner via an eight-centred transition state; thermolysis of 1-ethoxy-1-phenylbut-3-ene at 330–400° follows a first-order rate law, with Ea= 43 kcal/mol, ΔS‡=–10 cal K–1mol–1, affording ethylene, benzaldehyde, and propene as the only products, and the corresponding butyne derivative behaves similarly, giving allene as the only three-carbon fragment.