Transport of thailium(I) counterions was studied in solutions of poly(styrenesulfonic acid), PSSA, in mixed solvents by steady-state voltammetry at mercury microelectrodes. The diffusion coefficient values of T1(I) obtained in polyelectrolyte solutions were compared to those from solutions of p-toluenesulfonic acid, p-TSA, the simple acid, monomer analogue of PSSA. The dependence of the transport of T1(I) counterions on the dielectric constant, epsilon, of the solvent was investigated for such solvents as ethanol-, methanol-, and 1,4-dioxane-water mixtures over a wide range of epsilon-values. The diffusion coefficient of thallium(I) counterions in PSSA normalized by the diffusion coefficient in solution of the simple acid, p-TSA, was found to change proportionally to the change in the dielectric constant value of the solvent. This dependence is close to that predicted by Manning’s theory. The results were obtained over a wide range of concentrations of counterions and over a wide range of concentrations of various supporting electrolytes.