The uncatalvzed acetolvsis of bicyclo[2.1.0]pentane (1) leads to 47% cyclopentyl acetate and 53% cyclopentene. The reaction with p-toluenesulfonic acid in acetic acid is much more rapid and leads to 30% cyclopentyl acetate, 24% cyclopentyl tosylate, and 46% cyclopentene. The rate of acetolysis is decreased by a factor of 2.55 on going to AcOD as the solvent, indicating that proton transfer is at least partially rate determining. The deuterium distribution in the products of the reaction was determined, and extensive hydrogen migration was found. The preparation of 1-endo-5-d is reported, as well as the distribution of deuterium in the products of its acetolysis. Cyclopentyl acetate is formed mainly by a process involving inversion at both C1and C4 whereas cyclopentyl tosylate is formed largely by a process involving inversion at the site of proton attack, migration of the endo-5 proton, and capture of the tosylate anion. A detailed mechanistic scheme is proposed. The competition between strain relief and cation stabilization was examined with use of 5,5-dimethylbicyclo[2.1.0]pentane, and the latter was found to be more important. The rate of acetolysis of 1 is 88 times faster than that of bicyclo[3.1.0]hexane whereas a much larger rate ratio would have been expected if strain relief was an important factor in determining reactivity.