A simple strategy was developed based on a new reactive function- and a salt-containing new monomer, 4-vinylbenzyl dimethyl 2-(dimethylphosphino)ethyl phosphonium chloride (VDEPC), to obtain stable humidity-sensitive membranes. The major ingredient of a humid membrane is crosslinked polyelectrolytes obtained from copolymers of VDEPC/2-ethylhexyl acrylate (2-EHA) = 1/0, 4/1, and 2/1. Isothermal humidity absorption experiments were performed for the estimation of humidity-sensing materials. The crosslinked copolymers prepared from the reaction of VDEPC/2-EHA = 4/1 with 1,4-dichlorobutane showed an average impedance of 595, 39.1, and 3.9 KOmega at 30, 60, and 90% RH, respectively. Their hysteresis, temperature dependence, frequency dependence, and response time were measured. The reliability including water resistance and a long-term stability were estimated for the application of the common humidity sensor. (C) 2003 Wiley Periodicals, Inc.