Diluted olive mill wastewater (OMW) was subjected to direct current (DC) voltages (0.5-4.0 V) for hydrogen gas production with simultaneous chemical oxygen demand (COD) removal by electrohydrolysis. The highest cumulative hydrogen production (3020 ml) and hydrogen yield (2500 ml H-2 g(-1) COD) were obtained with 3 V DC voltage while the highest current intensity (80 mA), percent hydrogen (95%) in the gas phase, hydrogen gas formation rate (614 ml d(-1)), percent COD removal (44%) and energy conversion efficiency (95%) were realized with 2 V. Hydrogen gas production by electrolysis of water was negligible for all voltages. COD removal from OMW with no DC voltage application was usually lower than that obtained with DC power application. Hydrogen gas production by electrohydrolysis of OMW was proven to be a fast and effective method with simultaneous COD removal. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.