AbstractMethyl substitutents locatedperito bay‐region benzo rings of polycyclic aromatic hydrocarbons generally reduce or eliminate carcinogenic activity, most likely because such substituents force the hydroxyl groups of dihydrodiol intermediates to adopt preferentially pseudodiaxial, rather than pseudodiequatorial, conformations. Empirical force field calculations have been employed to examine the influences ofperiand other positional substituents on hydroxyl conformation, taking naphthalene, anthracene, and phenanthrene as model compounds. For the unsubstituted naphthalene and anthracene dihydrodiols, the pseudodiequatorial conformation is preferred. This preference is reduced by adjacent methyl substitution and reversed byperimethyl substitution. Because of steric crowding the phenanthrene bay‐region dihydrodiol preferentially adopts a pseudodiaxial conformation. The results are discussed in relation to recent experimental evide