Steady-state voltammetry for the reduction of hydrogen ion at a Pt microelectrode was carried out in an aqueous suspension of a monodisperse sulfonated polystyrene latex with hydrogen ion as counterion to the latex particles. The measured transport-limited current for the reduction of the hydrogen ion was related to the gradient-diffusion coefficient of the hydrogen ion over a wide range of concentration of inert, 1:1 electrolyte. The gradient-diffusion coefficient for hydrogen ion in the deionized, ordered latex crystal was less than 1/40th of the value found without the latex. Addition of a 100-fold excess of electrolyte (with respect to hydrogen ion concentration) resulted in a value for the gradient-diffusion coefficient close to that without the latex. A neutral molecule, 4-hydroxy-TEMPO, oxidized anodically in the suspension under these same conditions yielded transport-limited currents that were little affected by changes in salt concentration. This suggests that most of the hydrogen ions are electrostatically accumulated near the particles. This observation can be described in terms of an effective charge on the latex particles. The gradient-diffusion coefficient of hydrogen ion was found to be insensitive to the transition from an ordered crystalline phase of the latex to disorder upon the addition of electrolyte. The dependence of the gradient-diffusion coefficient on the concentration of 1:1 electrolyte agrees with the self-diffusion coefficient values predicted from the cell model using the nonlinear Poisson-Boltzmann equation and with that predicted by a simple, semiempirical model.