The electromotive force of the galvanic cell SCE/satd KCl/solution (2)/cationic membrane/solution (1)/anionic membrane/solution (2)/satd KCl/SCE has been measured for various electrolytes, using cellulosic cation- and anion-exchange membranes which behave as ideal or nearly ideal reversible electrodes. This method affords a simple means of determining the mean activities of numerous salts, including many for which other electrometric methods are not available, without the need to make extra-thermodynamic assumptions. The agreement between the values of the activities found and those tabulated in the literature is excellent in the range of concentrations studied less than or equal to 0.1 mol/kg, even for salts containing di- and tri-valent ions. This method is also suitable for measuring the mean activity of salts in solutions of polyelectrolytes as we report for the variation of 0.1 KCI activity with different amounts of the protein beta-lactoglobulin. The main reason for the high performance of this system as a reversible electrode for a variety of salts, lies in the fortunate mutual compensation of the water transport effect for the two cellulosic membranes, together with a negligible effect on the cell emf of the liquid film adhering to the membranes, circumstances which have been thoroughly discussed. The measurements were carried out at 25 +/- 0.05 degrees C.