Electrical dark and photo conductivities, sigma(d) and sigma(p), of vapor deposited films of alpha-sexithiophene, 4,4'-distyrylstilbene, and titaniumoxophthalocyanine were investigated as function of temperature, exposure time to oxygen of variable pressure, irradiation time and intensity. Molecular oxygen increases sigma(d) and sigma(p) by orders of magnitude. Cartful removal of elevated temperatures under high vacuum and assistance of irradiation will reduce both sigma(d) and sigma(p) to almost negligible values. From the rise and decay kinetics of conductivity and from luminescence quenching it is concluded that oxygen must be complexed with the film constituting molecules in order to act as separation center for electrical charge. The complex is formed already in the dark, but more efficiently upon irradiation. The interaction with oxygen is essentially reversible but leads in a side reaction also to irreversibly oxidized photoproducts. (C) 1998 Elsevier Science B.V.