Abstract Theoretical studies on the thermolysis in the gas phase of ethyl methyl and diethyl carbonates were carried out using ab initio theoretical methods at the MP2/6–31G(d) and MP2/6–311++G(2d,p) levels of theory. Two possible pathways were studied, one of them via a six‐membered cyclic transition state and the other one in two steps, via a six‐ and a four‐membered cyclic transition state, respectively, with the formation of an alcohol, ethylene and carbon dioxide. The nucleophilic substitution to give ethers was also studied. The results indicate that the reactions occur via a mechanism in two steps. The progress of the reactions was followed by means of the Wiberg bond indices. The kinetic parameters calculated for the studied reactions agree well with the available experimental results. Copyright © 2004 John Wiley & Sons, Ltd.