A hybrid isotherm equation for microporous solids was proposed in this paper by properly combining the Dubinin equation and the Nitta equation. The Nitta equation was incorporated to the "micropore-filling" Dubinin equation to accomplish three important goals, which the Dubinin equation fails to fulfill : (1) to provide the necessary Henry’s law at zero loading, demanded by thermodynamics, (2) to understand the strength of adsorbate-adsorbate interaction in micropore filling mechanism in micropores of various sizes, and (3) to demarcate the critical pore size above which the micropore filling mechanism is not operative. It was found that this demarcation pore is not intrinsic but rather it depends on the specific adsorbate-adsorbent systems as well as the operating conditions. For practical parameters used, it was found that this demarcation pore size is in the order of 16 Angstrom, compared to the upper limit of 20 Angstrom set by the IUPAC.