Isotherms at integer temperatures for xenon (210-260 K) and argon (120-142 K) on a mesoporous silica gel show that with increasing temperature (T) hysteresis decreases, vanishing at a capillary critical temperature (T(cc)), a temperature lower than the bulk critical temperature (T(c)). The desorption branches of all isotherms below T(cc) exhibit a nearly vertical step which corresponds to the first-order transition, capillary liquid to capillary gas. Using current theories of fluids in pores, a new method for the estimation of pore size distribution (PSD) is developed. The analysis requires a large number of isotherms at temperatures approaching T(cc), which, for many mesoporous adsorbents, involves operating at pressures up to ca. 3300 kPa. Both adsorptives give essentially the same PSD. This method of PSD analysis is also applied to published isotherms for xenon and carbon dioxide on Vycor porous glass. A less exact PSD analysis, requiring only a single isotherm at or near the normal boiling point of the adsorptive, is also developed.