Deiss [Electrochim. Acta 47 (2002) 4027] claimed that the diffusion coefficient of electrochemical systems is potential-independent and apparent dependency is just related to the experimental errors. In this commentary paper, based on solid arguments, it was demonstrated that it is not a correct conclusion. The diffusion coefficient is strongly potential-dependent as can be understood from statistico-mechanical standpoint. This is well known from more than a century with convincing theoretical formulations. Of course, the possible experimental errors mentioned by Deiss are true, but not responsible for potential dependency of the diffusion coefficient. (c) 2004 Elsevier Ltd. All rights reserved.