This work demonstrates that the activity of individual ions is a measurable property. It reiterates that reliable values can be obtained performing continuous runs in electrochemical cells formed by a half-cell ion selective electrode (ISE) and a standard single junction reference electrode, both immersed in the sample solution. A brief historical review shows that there are no fundamental reasons to believe that the activity of individual ions cannot be measured. A proposal is then made to clarify the meaning of terms in the definition of the electrochemical potential. Malatesta's claims [F. Malatesta, J. Solution Chem. 29 (2000) 771-779; F. Malatesta, Fluid Phase Equilb. 233 (2005) 103-109] against the method for measuring the activity of ions are rebutted. It is shown that these claims, based on the analysis of Taylor's cells, are not applicable to the systems used in our studies. Measurements of the ionic activity coefficient in potassium chloride solutions, that could be thought to resemble Taylor's cells, are discussed in detail. A study of iteration between the junction potential and the activity of ions is presented for the solutions of NaCl in water. The activity of ions is incorporated in the calculation of the junction potential used to re-calculate the activity of ions. Results show that convergence is attained in less than 10 iterations. The effect of the normalization convention is examined. Future lines of research are proposed. (c) 2005 Elsevier B.V. All rights reserved.