Thin crystalline films of phthalocyaninatozinc(II) (PcZn), and its derivatives tetrapyridotetraazaporphyrinatozinc(II) (TPyTAPZn) and naphthalocyaninatozinc(II) (NcZn); as well as of perylenetetracarboxilic dianhydride (PTCDA) and its derivative dimethylperylenetetracarboxilic diimide (MePTCDI), have been prepared by vapor deposition under UHV conditions. me electrical conductivity (sigma(d)) and photoconductivity (sigma(p)) of these films were measured on interdigitated microelectrode arrays (IMA) of PtSix on Si. The films were studied as prepared and during exposure to rising partial pressures of oxygen and ammonia. Changes of sigma(d) and sigma(p) with O-2 or NH3 exposure, as well as changes in their activation energies E(A), showed striking differences among-the studied compounds. Ground-state levels are derived from in-situ UPS experiments, excited state levels from luminescence measurements. The influence of the position of these energy levels on the electrical properties is discussed within existing models of molecular semiconductors, and the impact of these studies on the use of these materials in photoelectrochemical and solid state photovoltaic cells is discussed.