The distribution of ions in polymer-ionic surfactant solutions was described by the nonlinear Poisson-Boltzmann (PB) theory applied for cell model. On the basis of on the PB cell model, a new trace probe electrolyte method was developed to determine the surfactant aggregation number and the osmotic pressure of the solution. Potentiometric trace probe electrolyte (NaI) measurements were performed for the PVP-NaDS system, and the calculated aggregation numbers were compared to ones measured by fluorescence quenching methods. It was found that the mean aggregation number increases with the surfactant activity and that it is independent of the polymer concentration. The osmotic pressure of the PVP-NaDS system was measured by membrane osmometry.