It has been found that the visible and near infrared spectra of both the monoclinic and triclinic phases of lead phthalocyanine (PbPc) thin films are affected significantly by the presence of nitrogen dioxide. However, due to chemisorption of the nitrogen dioxide the films do not recover completely on evacuation but require healing in vacuum to effect a partial recovery. The monoclinic phase gives the largest response at 725 nm but the interaction appears to be irreversible and the original state cannot be recovered, whereas the triclinic phase using 510 nm incident radiation can be recovered almost completely and thus offers a more viable prospect of development of an optical gas sensor. Values for the band gaps of both phases are obtained from the absorption data as follows : monoclinic PbPc, 2.48 eV (in vacuum), 2.44 eV (in NO2); and triclinic PbPc, 2.49 eV (in vacuum), 2.46 eV (in NO2).