Poly(2-acrylamido-2-methylpropane sulfonic acid), AMPS, and its alkali salts were prepared. The impedances of these polymers were measured at various humidities and temperatures in the frequency range of 100-1 MHz. The results suggest that in the films of the alkali salts of poly(AMPS) containing 1-2 water molecules per AMPS monomer unit in the low and medium humidity ranges, the majority carrier is the alkali cations. For practical use, a humidity sensor, which is completely water-resistive, was prepared by forming an interpenetrating polymer network composed of a copolymer containing AMPS and hydrophobic poly(ethylene glycol dimethacrylate).