Results are presented (relative ion abundances, I.P. and A.P. measurements) to illustrate substituent effects in the mass spectra of some γ- and β-substituted methyl butyrates. It is argued that (i)‘sub-decomposition energy’ ions and (ii) competition in the source with fast fragmentation processes are important factors influencing parent/daughter ion ratios, which are then rationalised in terms of the variation of these two factors with substituent. In certain cases a substituent effect on the energy of activation of the transition state is also invoked. The variation in parent/daughter ratios at different energies is explained in terms of relative A.P.'s and frequency factors. Mechanisms are discussed for the elimination of MeOH from M⁺ and for the McLafferty rearrangement.