AbstractThe rates of gas‐phase thermolysis reactions of 2‐acetylcyclopentanone 1,2‐acetylcyclohexanone 2,N‐acetylcaprolactam 3,2‐acetylbutyrolactone 4,2‐acetyl‐2‐methylbutyrolactone 5, and 3‐acetyl‐2‐oxazolidinone 6 have been measured over a temperature range of 50 K. They undergo unimolecular first‐order elimination reactions for which logA= 11.7, 11.7, 11.2, 11.4, 11.5, and 11.1 s−1and Ea= 193.4, 189.5, 153.2, 201.0, 206.8, and 176.1 kJ mol−1, respectively. The effect of the ring size together with the effect of a heteroatom in the ring on the rate of thermolysis reactions for compound 1–6 is the subject of this work