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Post modification methods were used to synthesis MCM-41 with different pore sizes, modified by Al grafting and cation doping. Water adsorption characteristics were measured by dynamic water vapor adsorption and analyzed regarding equilibrium and kinetics. Results showed that Al grafting and cations doping modifications on MCM-41 changed water adsorption characteristics. Doped cations strengthened the water adsorption capacity at low RH following the order of Al-C12-Mg > Al-C12-Li > Al-C12-Na. Kinetics analysis indicated that capillary condensation process developed slowly due to low diffusion constant. But the adsorption transient uptake rates in capillary condensation sections were larger compared to other sections, Larger RH step not only resulted in larger uptake rate, but also promoted capillary condensation. Instead of calculating or measuring isotherms and diffusion coefficients respectively, parameters like transient uptake rate corresponding to the effects of both transport ability and adsorption capacity should be considered. [ABSTRACT FROM AUTHOR]