Ion selective electrodes have been used to measure the activity coefficients at 298.2 K of individual Na+ ions in aqueous solutions of Na2SO4 up to 3 molal, and of individual K+ ions in aqueous solutions of K2SO4 up to 0.69 molal. The activity coefficients of the sulfate anion were calculated using values of the mean ionic activity coefficients of the salts. Contrary to the behavior found in 1:1 electrolyte aqueous solutions, the activity coefficients of both cations have been found to be larger than the activity coefficient of the sulfate anion. A model proposed by Khoshkbarchi and Vera was used to correlate the data and model parameters are reported.