Hydrophobicity of a "water-tolerant" solid acid, CS2.5H0.5PW12O40, has been investigated by adsorptions of water and benzene. One water molecule was adsorbed irreversibly at 298 K per one proton of CS2.5H0.5- PW12O40 as well as H-ZSM-5 (Si/Al = 40), while H3PW12O40 formed a stable hexahydrate. From the ratio of adsorption area of benzene to that of water, the hydrophobicity of the solid acid surface was estimated to be in the order H-ZSM-5 (Si/Al = 628) > SiO2 > Cs3PW12O40 > H-ZSM-5 (Si/Al = 40) similar or equal to Cs2.5H0.5PW12O40 > SiO2-Al2O3 > Al2O3, where the adsorption area is defined as the product of the adsorption amount and molecular cross-sectional area. It is thus concluded that the surface of Cs2.5H0.5PW12O40 has a hydrophobic nature close to that of H-ZSM-5 (Si/Al = 40). This hydrophobicity is responsible for the high catalytic activity of Cs2.5H0.5PW12O40 for hydrolysis of ester in excess water.