Ion selective electrodes have been used to measure the activity coefficients at 298.2 K of individual ions in aqueous solutions of mixtures of NaCl and KCl up to 4 molal total chloride ion concentration, and of NaCl and NaBr up to 4 molal total sodium ion concentration. The experimental results show that the activity coefficient of an ion in the presence of different counterions depends on the nature and concentration of all counterions present in the solution. The results also show that the activity coefficients of the cations in an aqueous mixture are different from those of the anions. These effects can be considered as a corollary of the ion-ion and ion-solvent interactions. A modified form of the Fitter’s model, coupled with a mixing rule which distinguishes between the activity coefficients of the anion and the cation, has been used to correlate the experimental results.