Considerations are presented which indicate that the over-all result of adding a strong electron acceptor to a phthalocyanine film is to increase the quantum yield for charge-carrier production and to increase charge-carrier life- time. Thus, the ortho-chloranil negative ions act as efficient centers for the conversion of eleotromagnetic energy (which may be absorbed by phthalocyanine molecules situated many molecular diameters away) into charge carriers, and also act as effective electron traps. Similar, but apparently less effective, impurity centers existing in small amounts in undoped phthalocyanine could be responsible for its photoconductivity. Alternatively. the host molecules themselves could function as ionization centers. With pentacene and tetracene, it appears that the observed photo- and semiconductivity of the pure substances is largely due to the presence of impurity electron acceptors. Oxygen was not excluded during the measurements and could conceivably be the impurity electron acceptor. The effect of adding a strong reducing agent (electron donor) is to ionize these impurity acceptors, to prevent them from receiving electrons from the host molecule. and consequently to diminish both dark- and photo-conductivity. The present results indicate that the presence of electronaccepting impurities in organic semiconducting systems will increase the probability of utilization of charge carriers formore » chemical or biological processes by making recombination less probable and by increasing quantum yields. These factors would tend to counteract the effect of low-carrier mobilities. This may have significance for photosynthesis in plant materials, in which the efficiency of utilization of electromagnetic energy for chemical processes is high. 29 references. (C.H.)« less