The surface pressure and adsorption isotherm as functions of surfactant and salt concentration are determined for an ionized surfactant. This analysis differs from previous work in that the surfactant, while ionized in the bulk solution, is assumed to be nonionized at the surface. The analysis is applied to several soluble surfactants and provides a reasonable fit to experimental surface pressure data. With this analysis, the surface solution is found to be ideal for a number of surfactants, so one less parameter is required than is used by the regular solution theory model of Lucassen-Reynders (1).