Monomer 4-vinylbenzyl dimethyl 2-(dimethylamino)ethyl ammonium chloride (1) containing both ammonium salt and amine group was prepared and copolymerized with n-butyl acrylate (2) for the humidity-sensitive membranes. The copolymers 1/2 = 1/0, 4/1, 2/1 and 1/1 were cross-linked with 1,5-dibromopentane. Isothermal humidity absorption was measured for the estimation of humidity sensitivities. The cross-linked copolymer 1/2 = 2/1 showed an average impedance of 715, 42.1 and 3.1 kOmega at 30, 60 and 90% RH, respectively. Hysteresis, temperature dependence, frequency dependence and response time were measured to estimate as a humidity sensor. (C) 2002 Elsevier Science B.V. All rights reserved.