The distributions of dissociated protons in aqueous solutions of poly(vinylsulfonic acid) (PVS) were evaluated by measuring the visible light absorbance of small amounts of the pH indicator bromophenol blue (BPB) in coexistence with the PVS system under various conditions. The experimental results were compared with theoretical values calculated for the cell model, based on the Poisson-Boltzmann equation, and the classical two states model. The calculated values for the cell model reproduced the various tendencies of the experimental results and suggested first that free protons are distributed according to the Poisson-Boltzmann equation and second that the distribution of Na+ as a counterion of PVS gives slightly less osmotic activity than that of H+.