This work presents an in-depth study of the interrelation between the junction potential E-J and the activity of individual ions. The need to correct for the sign of the junction potential employed in previous publications is used to confirm that the calibration of the ion-selective electrodes largely cancels errors in the estimation of E-J. Revised values of the activity coefficients of individual ions are presented for all systems previously measured. A rederivation of Henderson equation is used to clarify the interpretation of its terms for asymmetric electrolytes. A new analytical equation to calculate E-J is derived and tested. This equation corrects for the nonideality of solutions and for the concentration dependence of the conductivity. The correction for nonideality is only necessary for the ions present in both the sample and the reference electrode solutions. The new equation represents a major improvement over the Henderson equation for E-J calculations. (C) 2004 American Institute of Chemical Engineers.