A phenomenological approach is used for deriving a difference equation for the density of reduced sites in films of electroactive polymers with conspicuous interparticle interactions. The approach involves simultaneous application of the lattice methods and Broensted's rule. This leads to generalization of equations for the surface layer that are known in theory of surface tension for nonelectrolytic solutions. Together with the Poisson equation for electric potential, the derived relationships make a complicated system of differential equations. Nevertheless, it can be solved by iterative methods. In the framework of this approach, expressions for the rates of injection of charge carriers into polymer films are obtained. Within a first approximation with regard to allowance for the forces of short-range interactions, their influence on the rates of injection of electrons and protons into a film is discussed.