The distribution of protons and the electrostatic potential in poly(allylamine hydrochloride) (PAAmHCl) aqueous solutions containing excess HCl were evaluated by measuring the visible light absorbance with a small amount of the pH indicator bromophenol blue (BPB), which coexisted in the system under various conditions. The experimental data, obtained as an apparent proton concentration ([H+](app)), were reproduced quantitatively by a cylindrical Poisson-Boltzmann cell model with only one fitting parameter. It was proved that [Hf],,, could be expressed as a product of the mean proton concentration and a dimensionless factor, which sensitively reflects the uniformity in the electrostatic potential not only for polyanionic systems such as poly(vinylsulfonic acid) solutions but also for the polycationic systems studied here. The surface electric potential of the polyion was determined from the experimental data and found to be constant with increasing [PAAmHCL] although it was expected to decrease from the cell model. This result suggested that the local conformation of the polyion chain is influenced much more by the ionic atmosphere than by the concentration of the polyion itself.