The effect of temperature on adsorption isotherms of Ar, N-2, O-2, C2H4, and CO2 on model mesoporous adsorbents (MCM-41) with different pore sizes (pore radius r(p) = 1.2, 1.4, 2.1 nm) has been measured. The results clearly show that the hysteresis loop shrinks with increasing temperature and eventually disappears at the hysteresis critical temperature (T-ch), which depends on the average pore size but lies substantially below the pore critical temperature (T-cp) A plot of (T-c-T-ch)/T-c against d/r(p) for all the gases examined, except for CO2, forms a single straight line passing through the origin, where d is the molecular diameter. This relationship predicts that the adsorption hysteresis never occurs in the cylindrical pore of radius smaller than twice the molecular diameter. On the other hand, the same plot for conventional adsorbents with an interconnected network of irregular pores seems to be smoothly linked to the data points of T-cp on MCM-41 rather than those of T-ch on MCM-41.