Gas-phase pyrolyses of carbonate esters, ${alpha}$- and ${eta}$-hydroxy esters and ${eta}$-hydroxy ketones have been studied theoretically by the AM1 MO method. Carbonate esters were found to decompose by two types of processes; in the reaction pathway involving an intermediate, the decomposition of the intermediate was rate-limiting, but direct pyrolyses were also possible via a six-membered cyclic transition state in which the methoxy oxygen attacks a hydrogen atom on the ${eta}$-carbon. The hydroxy esters and ketones were found to decompose in a concerted process involving a six-membered cyclic transition state. Successive methylation on the ${alpha}$- and ${gamma}$-carbon led to an increase in the reactivity in agreement with experiments.