Argon adsorption at 77 K on ordered mesoporous silicas (MCM-41, SBA-15) with pore diameters up to 9 nm and on disordered mesoporous silicas with larger pore sizes was investigated, allowing us to successfully develop a method to calculate pore size distributions (PSDs) for cylindrical pores with diameters below 15 nm. It was found that for argon at 77 K the capillary condensation pressure tends to gradually increase as the pore diameter increases up to about 15 nm, whereas larger mesopores do not exhibit capillary condensation. The capillary evaporation pressure was much less clearly related to the pore size, and the steepness of the desorption branch was not always correlated with the degree of structural ordering of a given adsorbent. A good correlation was found between the positions of capillan, condensation steps on nitrogen and argon adsorption isotherms at 77 K, whereas the correlation between the positions of capillary evaporation steps was much worse. Therefore, for argon adsorption at 77 K, the PSD calculations from adsorption branches of isotherms are feasible, but calculations from desorption branches are expected to be inherently difficult. The changes in the statistical film thickness as the pore size decreases were examined, and it was concluded that the common t-curve provides a satisfactory description of multilayer adsorption for pore sizes > 2 nm. The common t-curve and the relation between the diameter of cylindrical pores and the capillary condensation pressure were determined and subsequently used in PSD calculations based on a well-known algorithm. The resulting PSDs were in good agreement with those calculated from nitrogen data (77 K) and argon data (87 K), and thus the present work on argon adsorption at 77 K provided a firm foundation for the pore size analysis of pore sizes < 15 nm.