The photocatalytic oxidation of 2,5-dihydroxybenzoic acid (2,5-DHBA) has been performed using ZnO. The reaction was carried out using a UVA lamp and direct solar light for the catalyst activation. The response surface methodology was used in order to achieve optimal values of pH and proper amount of catalyst. The conditions found to be the best were pH 5.4 and 1.33 g L-1 of ZnO. Under such conditions 86% removal of 2,5-DHBA was attained after 15 min of reaction. Additionally, 80% and 70% depletion were found for total organic carbon (TOC) and chemical oxygen demand (COD) values, respectively. Moreover, under optimized conditions the catalyst does not show activity lost by photo-corrosion. The calculated electrical energy per order (EE/O) shows that the energy required for the decrease of the initial concentration in one order of magnitude is quite high (260 kW h m(-3)). However, it was also found that the reaction can be carried out using direct solar light making the process significantly cost-effective. (C) 2016 Elsevier Ltd. All rights reserved.