Conventional values of activity coefficients of ionic species, in this case sodium and chloride ions in aqueous NaCl, have been published, as if these were the experimental values of individual ion activity coefficients, although no experimental method is really able to provide these separate quantities but only the mean activity coefficients. A dangerous consequence is the possibility that scientists concerned with ionic solution theory may rely on such data, and derive wrong conclusions about the effectiveness of their theoretical work. This paper shows the reasons why the data in question have no relation to the ionic activity coefficients, which remain unknown, and are a mere artifact of the method adopted. (C) 2009 Elsevier Ltd. All rights reserved.