The Density Functional Theory B3LYP/6-31G* method is used to provide a detailed understanding of the origins of infra-and intermolecular (solvent) effects on the epoxidation of C-C double bonds by dimethyldioxirane (DMDO) in a model system, 2-methyl-2-butene. We found that the presence of hydrogen bond donor substituents, such as hydroxyl and amino groups, at the allylic position on the olefin leads to substantially decreased activation barriers for epoxidation. This effect is observed exclusively when a hydrogen bond interaction is present between the hydroxyl or amino substituent and the attacking DMDO molecule, and is not caused by inductive electronic effects of the substituents. An even more significant lowering of the activation barrier is seen when DMDO forms a hydrogen bond with methanol (representing a hydrogen bond donor solvent) in the transition state. Solvent polarity, studied using the SCIPCM model, influences the epoxidation barrier to a much smaller degree than do hydrogen bonding interactions.