Hydrogenation of benzene to cyclohexene has been studied over Ru/Al2O3 catalysts in the presence of water, Attention has been paid to the influence of reaction conditions, ruthenium precursors, and catalyst pretreatment. The reaction was carried out under kinetic control and under strong mass transfer limitation. In the presence of water the formation of cyclohexene cannot be simply ascribed to physical effects. Mass transfer limitations are, however, important to obtain high selectivities at high conversion. A pretreatment of the catalyst under hydrogen, in the absence of benzene, increases the selectivity to cyclohexene. No effect has been observed when the pretreatment is carried out under nitrogen. Catalysts prepared from RuCl3 are more selective with respect to samples prepared from chlorine-free precursors.