The gas-phase thermolysis reactions of ${alpha}$- and ${eta}$-methylated ethyl formates, Y = $CH-X-CHR_1CH_2R_2$ where X = Y = O or S and $R_1;=;R_2$ = H or $CH_3$, are investigated theoretically using the AM1 method. The experimental reactivity order is reproduced correctly by AM1 in all cases. The thermolysis proceeds through a six-membered cyclic transition state conforming to a retro-ene reaction, which can be conveniently interpreted using the frontier orbital theory of three-species interactions. The methyl group substituted at $C_{alpha};or;C_{eta}$ is shown to elevate the ${pi}$-HOMO of the donor fragment (Y = C) and depress the ${sigma}^{ast}$-LUMO of the acceptor fragment ($C_{eta}$-H), increasing the nucleophilicity of Y toward ${eta}$-hydrogen which in turn increases the reactivity. The two bond breaking processes of the $C_{alpha}$-X and $C_{eta}$-H bonds are concerted but not synchronous so that the reaction takes place in two stages as Taylor suggested. The initial cleavage of $C_{alpha}$-X is of little importance but the subsequent scission of $C_{eta}$-H occurs in a rate determining stage.