Transparent, electrically conductive composite films were obtained by vapor-phase polymerization of pyrrole into a poly(vinyl chloride) (PVC) matrix containing FeCl3. The effects of water and of organic solvents on the polymerization were investigated. It was found that the conductance of the films increases in the presence of both water vapor and of good solvents or swelling agents for PVC. The conductivity of the films rises with reaction time and so does the rate of increase with higher FeCl3concentration. The production of HCl was monitored by two methods, the precipitation of AgCl in an aqueous solution of silver nitrate and the increase in conductance of an aliquot of distilled water which had been placed into the reaction chamber. On the basis of our experimental results, we propose a reaction mechanism that explains the observed increase of conductance with time. A universal kinetic formula has been derived which relates the increase of electrical conductance to other parameters characterizing the boundary conditions of the polymerization.