Chemical trapping studies combined with optical and electron paramagnetic resonance measurements were employed to examine the mechanisms of the aqueous photochemistry of methyl-benzoquinone, (mBQ) at both low and high quinone concentrations. At low [mBQ], dimethylsulfoxide (DMSO) reacted with a photogenerated intermediate to form a methyl radical, but methane did not, thereby unequivocally excluding the hydroxyl radical. DMSO at concentrations between 50 mM and 2 M completely suppressed the formation of the hydroxylated quinone, while only slowing the formation of the hydroquinone, suggesting reaction with either the triplet state or an intermediate arising from the triplet. Addition of Cl-, a putative physical quencher of the triplet, inhibited the DMSO reaction both noncompetitively and competitively in a fashion similar to that observed previously with nitrite, formate, and salicylic acid, thus providing further evidence for a reactive intermediate distinct from the triplet. This intermediate is attributed to a water-quinone exciplex. The relative yield of the methyl radical from the DMSO reaction decreased with increasing [mBQ], suggesting that at high concentrations, a bimolecular reaction of the triplet with the ground-state quinone outcompetes the formation of the quinone-water exciplex.