We have studied the role that activated carbon surface chemistry plays in its photocatalytic activity under UV radiation and simulated solar radiation. In this study, we have used the commercial activated carbon Witco and five carbon samples obtained after its gamma radiation treatment, as this procedure allows the modification of the superficial chemistry of the carbon but not its physical properties. Sodium diatrizoate was used as a model compound for the degradation study. Its degraded percentage depends on the type of radiation and activated carbons used. To explain the carbon photocatalytic activity, we have demonstrated the formation of the electron hole as well as the formation of hydroxyl radicals and the superoxide anion in the UV/activated carbon and the Solar/activated carbon systems, and that the concentration of these oxidant species depends on the superficial chemistry of these materials. Moreover, the activated carbons, which have a smaller band gap allow higher concentrations of hydroxyl radicals, whilst those carbons with a larger band gap favour the formation of the superoxide anions. The degradation of the contaminant by UV/activated carbon and Solar/activated carbon systems has also been compared with the results obtained when the photocatalyst used is the traditional TiO2. The activated carbon efficiency as a photocatalyst is higher than TiO2 under UV radiation, which is due to the different concentrations of oxidant species produced in both systems. (C) 2017 Elsevier B.V. All rights reserved.