The relative stabilities of conformers of the bay-region tetrahydroepoxide of methylated chrysene have been calculated. From these calculations on tetrahydroepoxides, one infers that substitution of a methyl group in the same bay-region as the epoxide should destabilize both syn-diaxial and antidiequatorial bay-region diol-epoxide diastereomers with respect to the syndiequatorial and anti-diaxial diastereomers. The results of these calculations, together with recent experimental observations, suggest that the enhanced in vivo binding to DNA of the isomer having the methyl group and the epoxide in the same bay-region (1,2-diol-3,4-epoxide of 5-MeC) might be partially due to this destabilization of the syn-diaxial diastereomer. The carbocation delocalization energies associated with epoxide ring opening of the methylated bay-region tetrahydroepoxide isomers of chrysene are also given. © 1985 Elsevier Scientific Publishers Ireland Ltd.