Thick sublimed films of lead phthalocyanine which have been heated in air at 360 °C for 1 h show rapid conductivity changes on exposure to NO₂, with some slow components remaining. These slow components are more evident for high NO₂ concentrations, and increase in magnitude on prolonged exposure to the gas. Electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and X-ray diffraction (XRD) experiments to characterise the effects of heat treatment on the chemical and structural nature of the films are reported, and both morphological and chemical mechanisms for the rapid response are discussed. These results, together with previous secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy and calorimetric studies, and the absence of corresponding effects for planar copper phthalocyanine, strongly support selective reaction of strong adsorption sites as the most likely origin of the kinetic effects. The advantages and limitations of sensors using this material are discussed.