In this work we have studied the multilayer stepwise adsorption of gases on solid adsorbents based on the previously developed theory. It is shown that stepwise adsorption isotherms emerge from our theory if an ad hoc adsorption regarding the degree of occupation for each successive layer is abolished and the effect of lateral intermolecular interactions among adsorbate molecules is included. In addition to these the effect of vertical interactions has also been taken into consideration. It seems that the vertical interaction plays a role in deciding the shape and the position of steps in resulting isotherms. It is evident from this research that it is the lateral interaction that is responsible for stepwise adsorption as long as the adsorbent surface is uniform and temperature is sufficiently low.