AbstractThe kinetics of the isomerization of 1,1‐cyclopropanedicarboxylic acid1ato αcarboxy‐γ‐butyrolactone3ain D2O and in aqueous D2SO4(up to 56%) has been studied at temperatures up to 160°C.The pseudo first‐order reaction is progressively accelerated by increasing sulfuric acid concentrations and according to Bunnett's w, w* and ψ‐values, the rate‐limiting step is probably the nucleophilic attack of water on a cyclopropyl ring carbon atom of the protonated substrate.2‐Methyl‐1,1‐cyclopropanedicarboxylic acid1band 2,2‐dimethyl‐ 1,1‐cyclopropanedicarboxylic acid1cisomerize faster than1aby a factor of 31 and 80,000 respectively. The tremendous, rate‐enhancement of1ccan be explained by a rate‐limiting dissociation of a ring carbon‐carbon bond of the pr