Thin films of the α polymorphic modification of a nickel phthalocyanine (NiPc) which possess a high sensitivity to actions of gaseous media are deposited by the thermal vacuum deposition method. Indium tin oxide (ITO) continuous thin films on glass are used as substrates. On the basis of these films, the barrier structure ITO/NiPc/Al has been formed. We have found that there exists the electrogeneration effect in the ITO/NiPc/Al structure under the action of the gaseous ammonia medium. The appearance of a short circuit current and an open circuit voltage is observed only in the ITO/NiPc/Al structures that can be explained by redox reactions.