The adsorption energy and the equilibrium valley of various molecules adsorbed on a step of MgO(001) substrate are determined on the basis of interatomic potentials. The step acts as a trapping site for all molecules adsorbed on the lower terrace and as a barrier for molecules on the upper terrace. The height of the potential barrier increases or decreases when compared to the diffusion barrier on a perfect surface, depending on the electric nature of the molecule. While nonpolar admolecules experience a potential barrier at the upper step edge, this is a characteristic of the strong ionic substrate surface field to drastically reduce the potential barrier for polar molecules. The relaxation and the rumpling of the step ions change the adsorption energy by about 10%, resulting in an increase for a rare-gas adatom and in a decrease for all polar species, but they have little influence on the molecular orientation.