The reversibility of the adsorption isotherms of various gases has been studied on MCM-41 materials of various diameters, and their mixtures have been prepared by mechanical mixing in 1:1 weight ratio. As expected the results of the latter indicated a pore size distribution having two peaks. The isotherms were completely reversible with two capillary condensation steps for mixed samples whose component pore diameters were lower than 3.8 nm. The samples with one component having pore diameter above 3.8 nm indicated hysteresis associated with the larger pores, with the hysteresis loop found to close before the capillary evaporation from the smaller pore occurs. Empirically two critical sizes, one for absence of the condensation transition (D-CP) and one for absence of hysteresis (D-CH), have been recently identified, but quantitative study of the underlying phenomena has hitherto not been conducted for MCM-41. Various literature models, as well as a new model of the authors, were tested for explaining reversibility of the gas adsorption isotherms for MCM-41. It was found that the model of the authors utilizing the well-known tensile stress hypothesis is the most satisfactory among these alternatives.