Single-ion activities of H+ and Cl- in aqueous hydrochloric acid solutions up to 500 mmol.dm(-3) HCl have been determined, independently of each other, with reasonable accuracy, 95% confidence interval of log(10)gamma being +/- 0.012 at 25 degrees C, by use of an ionic liquid salt bridge inserted in a Harned cell. The geometric averages of those activities agree well with the literature values of the mean ionic activity of HCl determined with a Harned cell up to an ionic strength of 500 mmol.dm(-3). The agreement supports the assumption used in estimating the single-ion activities, that is, the liquid junction potentials on both sides of the ionic liquid salt bridge stay constant, within the confidence interval of the cell voltage, +/- 1 mV, over the concentration range of HCl between 10 and 500 mmol.dm(-3). The single-ion activity coefficient values of H+ and Cl- over the concentration range studied show fairly good agreement with the recent theoretical predictions by the "smaller-ion-shell" model proposed by Fraenkel (Mol. Phys. 2010, 108, 1435; J. Phys. Chem. B 2011, 115, 557), which fact is significant and implicative in physical and analytical chemistry of electrolyte solutions.