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Polymer-encapsulated cobalt phthalocyanine (CoPc) is a model system for studying how polymer–catalyst interactions in electrocatalytic systems influence performance for the CO2reduction reaction. In particular, understanding how bulk electrolyte and proton concentration influence polymer protonation and in turn how the extent of polymer protonation influences catalytic activity and selectivity is crucial to understanding polymer–catalyst composite materials. We report a study of the dependence of bulk pH and electrolyte concentration on the fractional protonation of poly(4-vinylpyridine) and related polymers with both electrochemical and spectroscopic evidence. In addition, we show that the fractional protonation of the polymer is directly related to both the activity of the catalyst and the reaction selectivity for the CO2reduction reaction over the competitive hydrogen evolution reaction. Of particular note is that the fractional protonation of the film is related to electrolyte concentration, which suggests that the transport of counterions plays an important role in regulating proton transport within the polymer film. These insights suggest that electrolyte concentration and pH play an important role in the electrocatalytic performance for polymer–catalyst composite systems, and these influences should be considered in both experimental preparation and analysis.