Abstract: The dielectric isotherms and the dielectric absorption curves of MgO, CaO, Ca(OH)_2 and TiO_2 have been measured at frequency from 400 Hz to 5 MHz in the present of water vapour (p/p_0= 0 to 0.75). Based on the Maxwell-Wagner-Sillars' theory, the general relation between adsorption and dielectric constant of these systems has been discussed. The differences in dielectric properties were observed for these solid oxides mentioned above. In addition, X-ray analysis showed that after adsorption of water vapour, the calcium oxide changed into crystallite calcium hydroxide. However, the dielectric properties of this sort of calcium hydroxide are different from general Ca(OH)_2/H_2O system. The results were explained preliminarily. It is concluded that from the studies of dielectric properties the useful informations about surface state of adsorbents and interactions between adsorbent and adsorbate can be obtained.