Ternary mutual diffusion coefficients measured by the Taylor dispersion method are reported for aqueous solutions of {ethambutol dihydrochloride (1) + HCl (2)} at 25 degrees C and various carrier solution compositions. Mutual diffusion coefficients estimated from limiting ionic conductivities using Nernst equations are used to discuss the composition dependence of the measured diffusion coefficients. H-1 NMR studies, combined with DFT calculations, confirm a fully extended conformation for the diprotonated form of the drug present under these conditions, and are consistent with an electrostatic mechanism for the strongly coupled diffusion of diprotonated ethambutol and HCl. (C) 2015 Elsevier Ltd. All rights reserved.