The adsorption of water at the clean and stepped MgO(100) surface was studied with the semiempirical molecular orbital method SINDO1. A cluster simulation was used for the MgO surface. Molecular and dissociative adsorption structures and their relative energies were calculated. Transition structures for dissociation and migration were determined. Pathways from the molecular adsorption structure on the clean surface to the dissociated structure at a step are presented.