Langmuir-Blodgett films of a symmetrically substituted copper phthalocyanine were deposited onto planar interdigital gold microelectrode arrays designed specially with a heating resistor. Measurements on this device showed that it could be used as a humidity sensor at room temperature, and as a gas sensor when it was heated by heating the resistor. Either as a humidity or gas sensor, it exhibits high sensitivity, rapid response and good stability. It has excellent selectivity to NH3 as a gas sensor.