Taylor dispersion has gained widespread popularity for measuring diffusion in liquids. The usual procedure is to inject small volumes of solution containing solute at concentration c + Delta c into carrier streams of composition c. Binary mutual diffusion coefficients D are evaluated from the Gaussian distribution of the dispersed solute measured at the outlet of a long capillary tube. As a result of strong dilution of the injected solute with the carrier solution, obtaining favorable signal-to-noise ratios for the measured profiles can require unacceptably large Delta c values for solutions with strongly composition-dependent diffusion coefficients or broad dispersion profiles produced by slowly diffusing solutes. For these systems, D can be reliably evaluated from error-function profiles generated by changing the solution flowing into dispersion tube from composition c - (Delta c/2) to c + (Delta c/2). There are no dilution factors, so Delta c can be orders of magnitude smaller than the values employed in conventional pulse-injection techniques. In the present study, the error-function dispersion technique is extended to measure coupled diffusion in three-component solutions using small Delta c initial conditions. A least-squares procedure is developed to calculate ternary mutual D-ik coefficients from profiles generated by changing the solution flowing into a dispersion tube from composition c(1) - (Delta c(1)/2) and c(2) - (Delta c(2)/2) to c(1) + (Delta c(1)/2) and c(2) + (Delta c(2)/2). D-ik coefficients are measured for aqueous solutions of Triton X-100 + poly(ethylene glycol) at 25 degrees C to study the interactions between nonionic micelles and polymers.