The rates and products of the reactions of (+/-)-7 beta ,8 alpha -dihydroxy-9 beta ,10 beta -epoxy-7,8,9, drobenzo[a]pyrene (1) and (+/-)-7 beta ,8 alpha -dihydroxy-9 alpha ,10 alpha -epoxy 7,8,9,10-tetrahydrobenzo[a]pyrene (2) in water and dioxane-water mixtures have been determined over a pH range wider than that of earlier studies. This study provides additional insight on the mechanisms of the pH-independent reactions of I and 2. The rate profile for reaction of 1 shows acid-catalyzed hydrolysis at pH <5, a rate plateau at pH 5-9.5, a negative inflection at pH 10-11.5, and a rate increase at pH > 11.5. The rate decrease between pH 10 and pH 11.5 is accompanied by a decrease in the yield of tetrols from 60% (pH 8) to 29% (pH 11.2) and is interpreted to be the result of a partial change in mechanism brought about by attack of hydroxide ion acting as a base to deprotonate a carbocation intermediate and regenerate I at pH > 10, thus reducing the contribution of the pathway for tetrol formation in which water attacks the carbocation. The rate profile for the reaction of 2 exhibits only a single rate plateau at intermediate pH, along with increases in rate at low and high pH because of second-order reactions of 2 with H+ acid HO-, respectively. The lack of a rate depression at pH > 10 and the product studies for the reaction of 2 in dilute sodium azide solutions suggest that the tetrol-forming reactions of the pH-independent reaction of 2 are concerted or near-concerted.