Surface properties of the organophilic silica gels were investigated by the adsorptions of argon at 77°K, water vapour at 288°K and n-heptane at 273°K and were also investigated by the heat of immersion of water and that of n-heptane on the silica gels which have been surface treated with methanol, ethanol, butanol, pentanol, hexanol, octanol, decanol, dodecanol and tetradecanol, respectively. It was noticed that (1) the shape of isotherms of water vapour adsorption showed the type II classified by BET on both native silica gel and silica gels treated by methanol or ethanol, whereas, it showed the type I on the silica gels treated by alcohol with longer carbon chain. The adsorbed amounts of water on the surface-treated silica gels were reduced remarkably in comparison with that on the native silica gel at the same relative pressure. (2) The argon adsorption yielded the reasonable surface area of the silica gels with either hydrophilic or organophilic surface. (3) The sum of the surface areas obtained from water vapour adsorption and from n-heptane adsorption on the organophilic silica gel showed a reasonable agreement with the surface area obtained from argon adsorption. (4) The surface of the treated silica gel was composed of both the organophilic surface due to surface group and the hydrophilic surface due to silanols which were either unreacted with alcohol or uncovered by surface groups. The appreciable amounts of surface silanol. were observed to be covered by adjacent surface group and this trend was significant with increase of the length of carbon chain in a surface group. (5) On the organophilic silica gels, the heat of immersion of n-heptane increased with an increase of length of carbon chain in a surface group. The heat of immersion of water showed a linear relation with the surface areas obtained from water vapour adsorption. t Studies on the Surface Treatment of the Ultrafine Powders. XV.