In dilute aqueous solution surface-active molecules aggregate to form micelles. This process has been described by a multiple equilibrium model which considers changes in the distribution of micelle aggregation numbers with concentration. Analysis of the colligative properties of dilute aqueous solutions of non-ionic surface-active agents has enabled the concentration of monomer to be determined as a function of the total solute concentration. The validity of the treatment has been confirmed by showing that these monomer concentrations are in good agreement with those deduced from nuclear magnetic resonance experiments, which do not depend on the assumptions employed in the thermodynamic method. Only one colligative property need be determined as a function of concentration in order to describe adequately the average aggregation numbers and free energies of micelle formation.