The dielectric behaviour in the TiO2–H2O adsorption system has been investigated at frequencies from 0.1 Hz to 5 MHz at room temperature. A large dielectric dispersion has been found from 0.1 to 104 Hz, which shifts to a higher frequency region when the coverage of physisorbed H2O increases. A simultaneous increase in conductance has been observed when the coverage increases. Thus, the dielectric relaxation observed at room temperature has been interpreted in terms of the interfacial polarization, which obeys the two-layer model and increases with increasing conductance. A mechanism is postulated for an enhancement of the conductance which takes place when H2O is adsorbed on TiO2.