The understanding of adsorption isotherms is important for the solution of a large class of industrial problems, many of which are of current research interest. The existence of stepped adsorption isotherms is often observed in some particular adsorption processes, being a distinction of mesoporous materials reflecting capillary condensation effects. However, such singularity is less observed at very low relative pressures. For instance, the presence of a substep on the argon adsorption isotherm of MFI at cryogenic temperature has been corroborated with a variety of experimental studies, but its origin is still unclear, and the interpretations are even contradictory. In this work, a new insight into this process is proposed by a combined approach using both experiments and molecular simulations.