The effect of temperature on adsorption isotherms of Ar, N-2, O-2, C2H4, and CO2 on mesoporous MCM-41 molecular sieves with different pore sizes (pore radius r(p) = 1.2, 1.4, 1.8, 2.1 nm) has been measured. The occurrence of adsorption hysteresis upon capillary condensation of these molecules in mesopores depends on their capillary critical temperatures (T-cc), which are far below the corresponding bulk critical temperatures (T-c). In accord with theoretical predictions of confined geometry effects on the criticality of fluids in narrow pores, a plot of (T-c - T-cc)/T-c against d/r(p) for Ar, N-2, O-2, and C2H4 seems to form a single straight line passing through the origin, where d is the molecular diameter. The data for CO2 deviates from this relationship. These results are compared with critical point shifts of fluids in conventional mesoporous adsorbents with an interconnected network of pores available in the literature.