This paper summarises recent progress in understanding the processes which lead to large increases in the semiconductivity of metal phthalocyanine films in the presence of even very small (sub-ppm) concentrations of nitrogen dioxide. A model, based on experimental data, for the kinetics of the adsorption process is described and its application in a microprocessor-controlled NO2-sensing instrument is outlined. Chemical and morphological modification of the surfaces of lead phthalocyanine films by heat-treatment at 360° C in air is shown to lead to smaller but faster responses to NO2and possible origins of these effects are discussed with reference to data from electron microscopy, XPS and FTIR spectroscopy. Measurements of the strength of gas-surface interactions using a novel fibre-optic interferometric calorimeter are discussed with reference to the above data, and suggest that adsorption of NO2does not lead immediately to charge-carrier generation.