Mesoporous silica gels having a narrow pore size distribution of around 2 nm in diameter were synthesized by a solvent evaporation method: C12 with pores of less than 2.0 nm and Cl 8 with pores of 2.2 nm. These synthetic mesoporous silica gels were evaluated from the viewpoints of adsorption capacity for water vapor and the temperature dependency of adsorption and desorption. C12 and C18 showed S-type adsorption isotherms with temperature dependency C12 exhibited a high effective adsorption capacity for water vapor of 0.11 kg-H(2)O/kg-silica gel in the range of the relative humidity Phi=0.29 (303 K) and Phi=0.21 (333 K). When a thermo-sensitive polymer (NIPA: N-isopropylacrylamide) was introduced into the C18 mesoporous silica gel, a drastic change was observed in the amount of water vapor adsorbed below and above the phase transition temperature (T(p)=306 K) of the thermo-sensitive polymer, resulting from a discontinuous change in the nature of the polymer from hydrophilicity to hydrophobicity. Application of a thermo-sensitive mesoporous silica gel/polymer gel with such properties to a desiccant air conditioning system is expected to reduce regenerating operation temperature considerably.