A theoretical description of ionic micellar solutions, which are considered as a mixture of two fully dissociated electrolytes with a common ion, is proposed. The solution is modeled within the framework of an improved primitive model, in which the solution permittivity as well as the charge and size of the solutes are allowed to vary with surfactant concentration. The MSA theory is used to estimate the electrostatic interactions, and the Carnahan-Starling approximation is employed to evaluate the hard-sphere contributions to the osmotic and activity coefficients. The experimental osmotic coefficients of sodium dodecyl sulfate (SDS) solutions are reproduced by using a value of 1.9 + 4C(T) nm for the micellar radius, CT being the total surfactant molar concentration. This is the only adjustable parameter in the theory, and the agreement with published estimations of the size of SDS micelles is excellent.