Intradiffusion coefficients for the ternary system heavy water-sodium hexanesulfonate (C6SNa)-pentaethylene glycol monohexyl ether (C6E5), in both the premicellar and micellar composition ranges, mere measured by the PGSE-NMR technique at 25 degrees C. Experimental data show the formation of mixed micelles and allow the determination of the cmc. In the premicellar composition range the monomer-monomer interactions are analyzed. The compositions of the aqueous and micellar pseudophase have been computed in the whole micellar composition range; they indicate that C6E5 has a larger tendency to form aggregates than C6SNa. The aggregation number and the free energy of micellization are calculated and interpreted in terms of interactions among the tensides in the micellar aggregates. The experimental results have been interpreted in terms of a regular solution model, and the interaction parameter beta is computed. beta increases by increasing the number of micellar aggregates in the system; this effect can be interpreted in terms of intermicellar interactions.