Under sulfur-deprived conditions, the metabolism of Chlamydomonas reinhardtii switches to the photoproduction of hydrogen. This process is sustained by both photosystem II-driven water splitting and by the fermentation of stored carbohydrates. We investigated the possibility of using diluted pretreated olive mill wastewaters (OMW), which contain organic acids and sugars, as a substrate on which to grow Chlamydomonas, in order to obtain suitable biomass to produce hydrogen. The cells grown on a mixture of pretreated OMW and TAP (tris-acetate-phosphate) (50% dilution) were found to be richer in carbohydrates and exhibited a greater production of hydrogen (150 ml H-2 l(-1) culture), compared to the control cells (100 ml H-2 l(-1) culture). In these cultures, the hydrogen production process was characterized by a shorter aerobic phase and a longer hydrogen-production period. The results offer a useful perspective for the utilization of olive mill wastewaters, which constitute an environmental problem, particularly in Mediterranean areas, and for increasing the output for hydrogen production with Chlamydomonas. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights