A theory of the concentration dependences of the chemical potential (the activity, the activity coefficient) of surfactants and the osmotic coefficient in a micellar system is formulated. The cases of nonionic and ionic surfactants are considered separately. In the first case, the analysis of concentration dependences is associated with the aggregation number of a nonionic surfactant, and the possibility of determining the aggregation number by the pattern of the osmotic coefficient isotherm is studied. The calculation formula for the degree of micellization above the critical micellization concentration (CMC) is derived more accurately. Based on published data, the aggregation numbers of surfactants were calculated in the approximation of an ideal system with allowance for deviations from the ideal behavior in the form of second virial coefficients, using the osmotic coefficient of aqueous alkyldimethylamine oxide solutions as an example. Since the aggregation number enters into the thermodynamic relationships under consideration in the form of its reciprocal value and only acts as a small correction, a reliable determination of the aggregation number from the osmotic coefficient isotherm requires a level of accuracy that is very difficult to achieve in experiments. In an ionic micellar system, the pattern of the osmotic coefficient isotherm is associated with the degree of counterion binding in a micelle. Calculation procedures for the degree of counterion binding from data on the osmotic coefficient were considered using an aqueous solution of N-dodecylnicotinamide chloride as an example. It was shown that accounting for deviations from the ideal behavior of a micellar system in terms of the theory of strong electrolyte solutions substantially affects the calculations of the degree of counterion binding; however, when the data on non-electrostatic deviations from ideal behavior are absent, we may only outline the boundaries of the region in which the true value of the degree of counterion binding lies.