A comprehensive study has been performed to investigate the response of cobalt phthalocyanine (CoPc) thin films to the effects of ambient NO2. Evaporated CoPc films were provided with interdigitated planar gold electrodes and exposed to NO2 concentrations of 5-100 ppm. In all cases the samples showed ohmic behaviour at room temperature, with current levels at 10 V applied voltage typically varying between 30 nA in the absence of NO2 to over 100 nA when exposed to 100 ppm NO2. The samples were also subjected to cycling of the NO, concentration, when the gas supply was alter lately switched on and off for periods of 20 min. The response to a step increase in NO, concentration, as well as the response when the NO2 was switched off were investigated. Initially the current increased rapidly, but this became slower after the exposure time increased. When the NO2 was switched off a sharp decrease in current was observed, followed by a slower decrease. These results were interpreted by reference to earlier work. The NO2 was initially rapidly chemisorbed onto the surface of the CoPc in the form of NO2 ions, which later slowly diffused into the bulk of the material. It was observed that at room temperature an equilibrium concentration could not be obtained within the time period investigated. Adsorption appeared to follow a Freundlich-type power-law isotherm.