The surface properties of silica gels treated with n-pentanol or benzyl alcohol accompanying with various numbers of surface adsorptive group were investigated from the adsorption. of argon at 77°K, heptane at 273°K and water vapor at 288°K, and also the heat of wetting into n-heptane or water at 288°K, respectively. The surface of the treated silica gels was confirmed to be composed of the hydrophilic and hydrophobic surfaces. In the formation of monolayer, the water vapor could not be adsorbed on the hydrophobic surface, but on the hydrophilic surface, whilst argon or heptane could be adsorbed on both surfaces. The n-heptane was found not to be adsorbed on the inner surface of the micropores with radius less than a molecular size of n-heptane. On this aspect. the surface areas of surface-treated silica gels obtained from argon, n-heptane and water vapor adsorption, ΣArHRSK, ΣhepHRSK, ΣH2OHRSK were represented by the following equations, respectively. ΣArHRSK =ΣArSK- (NCOH/NR) σOH·NR, ΣhepHRSK=ΣhepSK- (NCOH/NR) σOH·NR and ΣH2OHRSK=ΣH2OSK- (1+NCOH/NR) σOHNR·Where ΣXSK is the surface area of the native silica gel obtained from adsorption of X and NC OH, NR and σOH are the numbers of unreacted silanol caged in the micropores which were unable to afford adsorption site for any adsorbate, numbers of surface adsorptive group and the cross sectional area of silanol. These relations were confirmed to be reasonable in comparison with the experimental results. The pore structure of the surface-treated silica gels was discussed on the basis of the capillary condensation theory for the adsorption in the region of higher relative pressure. The heat of wetting on surface-treated silica gels, ΔHiX (erg/g of silica gel) in which X designates the wetting medium, was divided into two terms, i. e., interaction between X and hydrophilic or hydrophobic surfaces by using the relation, ΔHiX=ΔhiR-X ΣXR+ΔhiOH-X ΣXOH, where ΣXR and ΣXOH are the surface areas from the adsorption isotherms of X on the hydrophobic surface indicated by subscript R, and on the hydrophilic surface indicated by subscript OH which is mainly due to the unreacted surface silanols and afford the adsorption site for X, and also ΔhiR-X and ΔhiOH-X are the heat of immersion per unit area of hydrophobic surface indicated by superscript R-X, or hydrophilic surface indicated by OH-X. Thus obtained ΔhiR-X and ΔhiOH-X showed reasonable values, comparing with the available data on purely hydrophilic or hydrophobic surfaces. Accordingly, it was pointed out to be still questionable that the heat of wetting on the composed surface of either hydrophilic or hydrophobic was usually obtained by dividing the liberated heat by the apparent surface area obtained from the nitrogen or argon adsorption.