UV LED technology has revolutionized the photocatalytic processes due to their significant advantages over traditional mercury-based illumination sources, specifically its higher energy efficiency. However, the use of LED also introduces important changes in the light distribution achieved inside a photocatalytic reactor that has to be considered. In this study an exhaustive comparison of three different UV-A sources (a mercury fluorescent lamp, a 8-LED based system and a 40-LED based system) with different light distribution has been carried out. Theoretical distribution of the light was modeled in Ansys Fluent v14.5. The results of photocatalytic activity for methanol oxidation show that a homogeneous light distribution allows achieving a higher photonic efficiency. The diffuse and uniform emission of the fluorescent mercury lamp partially compensates its lower energy efficiency, leading to similar results than the 8-LED system. This fact can be explained taking in consideration that electron-hole recombination is enhanced in the areas with higher radiation intensities, decreasing the overall efficiency. In the case of the 40 LED due to the improvement of the light homogeneity and energy efficiency, higher reaction rates per kWh were achieved. These results show that despite the advantages of LED, if light distribution is not optimized it can result in lighting systems less effective than traditional ones. On the opposite, for bacterial inactivation the results show that there is no clear difference when using different lighting sources. The existence of a highly non-uniform radiation field with regions of the reactor with very high intensities seems to enhance the efficiency of the direct bacterial inactivation when LED are used, compensating the decrease in the charge transfer efficiency of the semiconductor based photocatalytic process. (C) 2017 Elsevier B.V. All rights reserved.