Photochemical processes affect the fate of spilled oil in the environment, but the relative contribution and kinetics of these degradation pathways are not fully constrained. To address this problem, we followed the weathering of No. 6 fuel oil by periodically sampling rocks covered with a film of oil from Buzzards Bay, MA after the April 2003 Bouchard 120 oil spill. Two sets of polycyclic aromatic hydrocarbon (PAH) isomers, benzo[a]pyrene (BAP) and benzo[e]pyrene (BEP), and benz[a]anthracene (BAA) and chrysene (CHR), were found to have very different disappearance rates in spite of their close structural similarity (kBAA/kCHR approximately 2.0, kBAP/kBEP approximately 2.2). This well-documented phenomenon is suspected to arise from differing capacity for direct photoreaction in the oil film. To investigate the validity of this assumption, we developed a model to estimate the contribution of direct photolysis to the loss of these PAHs from the oil. Newly determined PAH quantum yields demonstrate that the efficiency of phototransformation in hydrophobic media are 2 orders of magnitude lower (Phi' approximately 10(-5)) than in aqueous systems, and the thickness and light-attenuating properties of the oil film reduce the potential for photoreaction by up to 2 orders of magnitude. Given these limiting factors, direct photolysis cannot account for the complete removal of these PAHs (except BAP). Additional results suggest that singlet oxygen photodegradation pathways are not favored in hydrophobic media, as they are in some mineral-associated and aqueous systems. Our results indicate that photomediated reactions with other compounds in the oil mixture were responsible for PAH photodegradation in the oil film.