Electrical conductivity and optical absorption are studied simultaneously for lead phthalocyanine films exposed to nitrogen dioxide to assess their relative merits for the development of a PbPc-based thin film NO2 sensor. Additionally, frequency variations of an oscillating quartz crystal during NO2 adsorption together with Fourier Transform Infrared analysis have been used to investigate diffusion and bonding of NO2 at the surface and in the bulk of the films. The results indicate that sensors based on electrical conductivity offer a better response and recovery provided that exposure is limited. Long-term exposure prevents film recovery. This is related to the relatively slow in-diffusion of the NO2 into the bulk, leading to an NO2:PbPc molecular ratio of 4:1 in the bulk after extended exposure.