Thin films of a metal-free octahexyl-substituted phthalocyanine were prepared by spin coating. The films were smooth and showed optical absorption spectra similar to those of films of a structurally related compound prepared by the Langmuir-Blodgett technique, A novel gas sensor has been fabricated by incorporating these films into the gate electrode of a metal-oxide-semiconductor field-effect transistor. On the application of a gate voltage, a delay was observed in the response of the drain current. This delay characteristic was found to depend on the concentration of NO2. Results are presented showing that the device can detect reversibly concentrations of NO2 down to 2 ppm at room temperature.