Taylor dispersion and differential refractometry are used to measure ternary mutual diffusion coefficients for aqueous sodium dodecyl sulfate (NaDS) + sodium octanoate (NaOct) solutions at 25 different compositions at 25 OC. Diffusion of the mixed surfactants is strongly coupled. In particular, each mole of NaOct co-transports up to 2.5 mol of NaDS. The results are interpreted by using Nagarajan's molecular thermodynamic model of mixed surfactant solutions to estimate average surfactant aggregation numbers for the mixed micelles. In addition, Nernst-Planck equations are used to relate the measured fluxes of the total NaDS and total NaOct components to the fluxes of free DS- and Oct(-) surfactant ions, free Na+ counterions, and micelles of local-average composition. The analysis shows that the large coupled flows of NaDS driven by NaOct gradients are caused by the migration of charged NaDS-rich micelles in the electric field generated by the diffusion free Na+ and Oct- ions.