Ternary mutual diffusion coefficients (D-11, D-22, D-12 and D-21) measured by the Taylor dispersion method are reported for aqueous solutions of {levodopa ((L)- dopa) + HCl} solutions at 25 degrees C and HCl concentrations up to 0.100 mol - dm (3). The coupled diffusion of (L)-dopa (1) and HCl (2) is significant, as indicated by large negative cross-diffusion coefficients. D-21, for example, reaches values that are larger than D-11, the main coefficient of (L)-dopa. Combined Fick and Nernst-Planck equations are used to analyze the proton coupled diffusion of (L)-dopa and HCl in terms of the binding of H+ ions to (L)-dopa and ion migration in the electric field generated by (L)-dopa and HCl concentration gradients. (c) 2013 Elsevier Ltd. All rights reserved.