New phosphonium salt-containing monomer, (vinylbenzyl) tributyl phosphonium chloride (1), was prepared and polymerized for a humidity-sensing film. The humidity-sensitive membranes were composed of copolymers with different contents of 1 and styrene (2) or n-butyl acrylate (3) (1/2 or 1/3 = 1/0, 4/1, 2/1 and 1/1). When the impedance dependencies on the relative humidity of the copolymers were measured, the impedance increased with an increase in the content of 2 or 3. The impedance ranged from 10(7) to 10(4) Omega between 20% RH and 95% RH, which was required for a humidity sensor operating at ambient humidity. Temperature dependence, frequency dependence, hysteresis and response time were also measured and reliability was evaluated.