The gas phase pyrolyses of eight esters have been studied by MNDO-MO method. In the ethylformate pyrolysis, ${alpha}$-methylation had a steric releasing effect whereas ${eta}$-methylation had a steric crowding in the transition state; the latter, however, is over-compensated by a greater electronic repulsion resulting in a net steric releasing effect. Considerations of formal charges and geometrical changes involved in the activation led us to propose a pyrolysis mechanism in which a preequilibrium of acidic proton transfer is followed by the rate-limiting bond polarization of $C_{alpha}$-O bond in a cyclic transition state.