A composite film consisting of conducting poly(o-phenylenediamine) (PoPD) and insulating poly(vinyl alcohol) (PVA) has been prepared in a proportion which possesses satisfactory mechanical strength and characteristics favorable for a humidity senor. The electrical conductivity and PoPD content curves were measured, and it was found that the conductivity Of the composite film rose sharply at about 0.1 volume percent (v/o) of the PoPD content, reaching a value of similar to 10(-2) S cm corresponding to that of PoPD itself. The change in conductivity was as large as four orders of magnitude, which was attributed to the formation of networks that enable the insulating composite to reveal the electrical conduction. The conductivity of the composite film was linearly related to the humidity, extending from 2.5 x 10(-5) to 1.5 x 10(-1) S cm(-1) between the dry and wet states. Measurements of the conductivity were not attended with hysteresis in the moistening or desiccating runs. The change in conductivity was caused by a shift of the equilibrium between the conducting and the insulating PoPD depending on the atmospheric humidity in which, e.g., in the drying process the reaction of the protonated and conjugated PoPD with the OH group of PVA occurred to form the insulating PoPD and a water molecule resulting in the decrease of the conductivity of the composite film.