A novel temperature-switchable vapor sensor was prepared by radiation-induced polymerization of N-isopropylacrylamide (NIPAAm) in an alcohol solution of calcium chloride (CaCl2). The conductivity of the sensor material largely increased in water and ethanol vapors, but decreased against acetone, tetrahydrofuran, and chloroform vapors. The data showed a high level of reproducibility. In addition, the conductivity increased with the rise of temperature in ethanol vapor, showing a slight hysteresis, while against water vapor it exhibited a tendency to decrease with temperature accompanied by a large hysteresis. On the other hand, a densimeter to determine the water concentration in acetone solution was designed based on the opposite response of polyelectrolyte to two vapors, acetone and water, resulting in a linear responsiveness at low water concentration.