Moments analysis of Taylor dispersion profiles is used to measure ternary mutual diffusion coefficients (D-ik) for dilute aqueous sodium dodecyl sulfate (SDS) + dodecylsulfobetaine (DSB) mixed surfactant solutions at 25 C and total surfactant concentrations from 10 to 30 mmol dm(-3). The fluxes of the SDS(1) and DSB(2) components are strongly coupled by the formation of SDS-DSB mixed micelles. At several compositions; cross-coefficient D-12 or D-21 is larger than main coefficients D-11 and D-22. More remarkably, D-22 is negative at solute fractions of SDS from 0.55 to 0.75, reaching a minimum value of -0.056 x 10(-9) m(2) s(-1) at 13 mmol dm(-3) SDS + 7 mmol dm(-3) DSB. This surprising result means that DSB concentration gradients drive "incongruent" fluxes of DSB, from lower to higher DSB concentrations. A possible explanation for the incongruent diffusion of DSB is suggested on the basis of the electric field generated by gradients in the surfactant concentration. Negative D-ii values have been reported previously for concentrated, strongly nonideal aqueous acetic acid + chloroform solutions near a consolute point. Because the SDS + DSB solutions are dilute and far removed from phase boundaries, incongruent diffusion might be considerably more common than previously assumed.