AbstractThe kinetics of the gas‐phase pyrolysis of ω‐chlorocarboxylic acids were examined in a seasoned static reaction vessel and in the presence of at least twice the amount of the free radical inhibitor cyclohexene or toluene. In conformity with the available experimental data on rate determination, these reactions proved to be unimolecular and obeyed a first‐order rate law. The presence of the primary chlorine leaving group in Cl(CH2)nCOOH (n= 1–4) showed a change in mechanism from intramolecular displacement of the Cl leaving group by the acidic hydrogen of the COOH to anchimeric assistance of the carbonyl COOH to the CCl bond polarization in the transition state. This mechanistic consideration is nearly the same for the series of 2‐, 3‐, and 4‐chlorobutyric acids. The chlorine atom at the 2‐position of acetic, propionic and butyric acids is dehydrochlorinated through a prevailing reaction path involving a polar five‐membered cyc