The aim of the present study was to investigate the possibility for fermentative biological hydrogen production from olive pulp at 35 degrees C, using a 500 ml CSTR-type digester. The mesophilic reactor was inoculated with a hydrogen-producing culture and operated at hydraulic retention times of 30, 14.5 and 7.5 h. Also, batch experiments were carried out in order to assess the overall potential for hydrogen production and to investigate the effects of inoculum to substrate ratio and of hydrogen partial pressure on hydrogen production. It is shown that fermentative bio-hydrogen production from olive pulp is feasible resulting in 2.8-4.5 mmole H-2 per g of carbohydrates consumed in the reactor. The hydrogen potential of the olive pulp measured from batch experiments lay between 4 to 6 mmole H-2 per g of carbohydrates depending on the hydrogen partial pressure in the vials.