Taylor dispersion and dynamic light scattering techniques were used to measure the ternary diffusivity matrix [D] and the micelle gradient diffusion coefficient, respectively, in crowded aqueous solutions of decaethylene glycol monododecyl ether (C12B10) and decane. The results indicate that C12B10 diffused down its own gradient with the micelle gradient diffusivity while decane diffused down a decane gradient at a much slower rate. Furthermore, strong diffusion coupling, comprising decane diffusion down a surfactant gradient and surfactant diffusion up a decane gradient, was also observed with cross diffusivities that were on the order of or larger than the main diffusivities. Measurements of the micelle aggregation number, hydration index, and the hydrodynamic radius, obtained using both static and dynamic light scattering methods, indicate that decanecontaining micelles interacted as hard spheres and had radii and aggregation numbers that increased linearly with the molar ratio of solute to surfactant. A theoretical model, developed using Batchelor's theory for gradient diffusion in a polydisperse system of interacting hard spheres, was effectively used to predict [D] with no adjustable parameters. A comparison with the theory indicates that decane diffused down its own gradient by micelle self-diffusion while surfactant diffused down a surfactant gradient by micelle gradient diffusion. It is also shown that intermicellar interactions drove decane diffusion down a C12B10 gradient by a volume exclusion effect while an increase in the micelle aggregation number and hydrodynamic radius with decane was necessary to drive surfactant diffusion up a decane gradient.