Abstract: By the use of a precision automatic adiabatic calorimeter we have measured the heat capacities for water/silica adsorption system with various adsorbed amoun- ts over the temperature range from 200-320 K.The results obtained show that when the amount of adsorbed water makes the surface coverage (θ) larger than 1, the peak for phase transition of adsorbed water will appear on the C_p-T curves.This means that aggregate of water molecules adsorbed on the surface of silica has for- med. On the other hand when θ<1, no phase transition is observed as the temperature of the system rise and then the C_p—T curves are smooth. These phemomena are in coincidence with molecules on silica surface is weaker than that on the γ-Al_2O_3, the peak profiles for water/silica is sharper than that for H_2O/γ-Al_2O_3 even if the two adsorption systems have the same amount of adsorbed water. The rise in the peak temperature with increase of amount of adsorbed on the surface of silica could become buk water easier as compared with the water adsorbed on the γ-Al_2O_3. Besides, the heat capacities for silica without any adsorbed water over the temperature range from 200 K to 300 K have been obtained by extrapolating the C_p—T curves with various adsorbed amounts at the same temperatures.