This work reports individual ion activity coefficients in aqueous solutions of mixed 1:1 electrolytes with a common ion for the systems NaNO3 + KNO3, NaCl + KCl, and NaCl + NaBr at 298.15 K. These values were obtained from electrode potential measurements of ion-selective-electrodes (ISEs) of the common ion against an Ag-AgCl single-junction reference electrode filled with 4 mol . kg(-1) KCl solution saturated with AgCl. For the NaNO3 + KNO3 and NaCl + KCl systems, experiments were carried out at cation molal fractions of sodium of 0.75, 0.5, and 0.25, and for the NaCl + NaBr system, experiments were carried out at an anion molal fraction of chloride of 0.5. For NaNO3 + KNO3, single ion activity coefficients were obtained up to 2.4 mol . kg(-1) total nitrate concentration, and for the NaCl + KCl and NaCl + NaBr systems, up to 4 mol . kg(-1) of total chloride and sodium concentrations, respectively. The Henderson equation was used to estimate the value of the liquid-junction potential.