The effect of biomass water leaching on H(2) production, as well as, prediction of ash thermal behavior and formation of biomass tar during high temperature steam gasification (HTSG) of olive kernel is the main aim of the present work. Within this study raw olive kernel samples (OK(1), OK(2)) and a pre-treated one by water leaching (LOK(2)) were examined with regard to their ash fouling propensity and tar concentration in the gaseous phase. Two temperatures (T = 850 and 950 degrees C) and a constant steam to biomass ratio (S/B = 1.28) were chosen in order to perform the steam gasification experiments. Results indicated that considering the samples' ash thermal behavior, it seemed that water leaching improved the fusibility behavior of olive kernel; however, it proved that water leaching does not favour tar steam reforming, while at the same time decreases the H(2) yield in gas product under air gasification conditions, due to possible loss of the catalytic effect of ash with water leaching. (C) 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy.