Exposure of single crystals of zinc and lead phthalocyanine to low pressures (1–10 Pa) of NO2 and BF3 enhances the photoconduction by factors of ca. 100 and 10, respectively, and the photoconduction action spectra change to resemble the absorption spectrum, rather than its inverse as in vacuo. In air, similar changes in action spectra, but smaller changes in magnitude, occur. Additional slow increases for lead phthalocyanine in air are consistent with oxygen diffusing into the crystals. Photocurrent is proportional to (light intensity)n, with n= 0.54–0.62 in vacuo, 0.34–0.40 in air and 0.20–0.27 in NO2. Charge carrier generation mechanisms involving exciton dissociation at surface sites occupied by adsorbed gas molecules are discussed. Singlet photoconduction in perylene crystals is inhibited by low pressures of NO2 and BF3, and spectroscopic evidence is presented for reaction of NO2 with surface layers of perylene, forming nitroperylene. A new photoconduction response at higher NO2 pressures (103 Pa), centred at 16 200 cm–1, is consistent with carrier generation following excitation of a surface perylene–NO2 charge-transfer complex. This is reversible on evacuation and of similar magnitude to the vacuum photoconduction. The different behaviours of perylene and phthalocyanines are discussed in terms of the greater mobility of electronic excitation energy in phthalocyanines. Photoconduction, although more sensitive to low gas concentrations than semiconduction, has less potential for gas detection due to the poor reversibility of the effects.