This paper reports the first complete theoretical study of substituent effects on the mechanism of the Baeyer– Villiger (BV) reaction in non-polar solvents taking into account the lowest-energy mechanism that has been proposed for this rearrangement which is non-ionic and fully concerted. The BV reaction of p-substituted acetophenones, p-XC6H4COCH3 (X = NO2, CN, H, CH3, OCH3), with performic (PFA) and trifluoroperacetic (TFPAA) acids, catalyzed by formic (FA) and trifluoroacetic (TFAA) acids, respectively, using the MPWB1K functional and the 6-311G(d,p) and 6-311++G(d,p) basis sets, are studied. Solvent effects are taken into account by means of the PCM continuum model using dichloromethane as solvent. Electron-donating substituents on the aryl group have a relatively small activation effect on the first step, but a pronounced activation effect on the second to the point of being able to change the rate-determining step (RDS) of the reaction, as observed in the case of p-methoxyacetophenone with TFPAA acids. After analyzing the changes in Gibbs free energy of activation, geometrical parameters, and charge distributions of the transition states (TSs), explanations are provided for the two distinct effects that substituents on the ketone have on the kinetics of the addition and migration steps of the BV oxidation. The effect of the acid/peracid pair used is also discussed. Copyright © 2008 John Wiley & Sons, Ltd.