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.