Logarithm of electrical conductivity of poly(o-phenylenediamine)/poly(vinyl alcohol) composite is known to be proportional to relative humidity covering more than five orders of magnitude. Such a humidity-sensing system has been analyzed by ac impedance spectroscopy. Two different equivalent circuits for the humidity-sensing process are proposed in the humidity region lower and higher than 70% RH. In the former, the film impedance (R-film, C-film) and the interfacial impedance (R-ct, C-dl) are combined in series. The first semicircle on the Nyquist plot corresponds to the impedance of the composite him, and the start of the second semicircle is related to the interface impedance. In the latter, the Warburg diffusion is involved, which is attributed to the diffusion of the protonic acid from the insulating polymer unit to the whole composite. The logarithmic value of R-film changes Linearly over more than eight orders upon the variation of relative humidity from 20 to 95% RH, and the capacitive contribution (C-dl) at the film/electrode interface becomes appreciable for the composite films with larger thickness (>50 nm) and/or with larger weight ratio of PoPD to PVA (>0.25). (C) 2001 The Electrochemical Society. All rights reserved.