Ternary mutual diffusion coefficients (D(ik)) are reported for aqueous solutions of dodecylsulfobetaine (1) + hexadecylsulfobetaine (2) mixed zwitterionic micelles. Cross-coefficient D(12) reaches values almost as large as the main D(ii) coefficients, indicating strongly coupled fluxes. As the surfactant concentrations are raised and the extent of micelle formation increases, values of the determinant D(11)D(22) - D(12)D(21) of the D(ik) matrix drop sharply. Previous studies have shown that the thermodynamic stability constraint mu(11)mu(22) - mu(12)mu(21) >= 0 on the concentration derivatives mu(ik) = partial derivative mu(i)/partial derivative C(k) of the chemical potentials causes D(11)D(22) - D(12)D(21) to vanish at critical points and phase separation boundaries. Consequently, the cross-coefficients become similar in magnitude to the main coefficients. Prompted by the insight that might be gained by demonstrating an analogy between diffusion in micelle solutions and diffusion near phase separation, a model of mixed-micelle formation based oil multiple monomer association equilibria [nA(1) + mB(1) = A(n)B(m)] is used to show that mu(11)mu(22) - mu(12)mu(21) drops almost to zero with increasing extent of micelle formation. This result, which is generalized to other association colloids, points to thermodynamics as the underlying cause of strongly coupled fluxes, incongruent diffusion, and other remarkable features of diffusion in solutions of mixed micelles, solubilizates, and microemulsions. A molecular interpretation of diffusion in these systems, consistent with the Gibbs-Duhem and Onsager reciprocal relations, is developed by relating the D(ik) coefficients and the Onsager L(ik) transport coefficients to the concentrations and the mobilities of the free monomers and various A(n)B(m) species.