The dielectric behavior of Ag2O samples with different amounts of adsorbed H2O was investigated in the temperature range from 308 to 200 K and in the frequency region from 0.1 Hz to 5 MHz. A large relaxation is observed in these ranges, and it is explained by the interfacial polarization due to the heterogeneity of the system. The characteristic frequency (f(m)) for this relaxation is lowered with decreasing temperature until a complete monolayer is established. However, such a behavior of f(m) in the region beyond the surface coverage of ca. 1 reveals a new specific feature; the f(m) value gives a maximal frequency at around 278 K. The cause of this behavior is explored through the measurements of the differential adsorption heat, water content, and Ciausius-Clapeyron plots (ln p vs 1/T). As a result, a new phenomenon observed in the Ag2O-H2O system can be explained by considering that the continuous phase change occurs at around 278 K in the adsorbed layer higher than the monolayer; the cluster-like structure formed on the OH groups at a temperature higher than 278 K is transferred to the two- or three-dimensional network structure formed by H-bonding between adsorbed H2O molecules in the lower temperature region.