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This paper probes the role of hydroxyl radicals (center dot OH) generated by H2O2 photolysis on graphene oxide (GO) phototransformation, under simulated sunlight. It focuses on comparing the photoreaction of GO with (indirect) and without (direct) added H2O2 under simulated sunlight conditions. The biomarker responses of fish epithelial cells in in vitro assays of parent GO and GO photoreacted with H2O2 and their interaction with model biomembranes are also compared. GO was found to be far more extensively photodecomposed in the presence of H2O2, with similar to 85% of the initial carbon content converted to CO2 during 48 h of irradiation. Direct and indirect photoreactions occurred concurrently in GO samples containing H2O2, and indirect photoreaction accounted for similar to 70% of GO conversion to CO2. Reaction with center dot OH causes increases in the concentrations of carboxylic acid groups of photoreacted GO and low molecular-weight (LMW) species as part of the intermediate photoproducts. Compared to parent GO, intermediate photoproducts exhibited reduced interaction with model cell membranes and altered biomarker responses. Kinetic analysis extrapolating our data to conditions prevalent in sunlit surface waters predicts that initial GO photoreaction is dominated by direct photolysis, while indirect photo reactions involving center dot OH determine subsequent conversion of intermediate GO photoproducts to CO2. (C) 2016 Elsevier Ltd. All rights reserved.