The thermodynamic behavior of the aqueous KCl + K2SO4 electrolyte has been studied on the basis of potential difference measurements on the reversible cell K-amalgam \KCl (m(KCl)) + K2SO4 (MK2SO4) \AgCl\Ag\Pt. The mean molal activity coefficients gamma(KCl) of KCl at molalities m(KCl) and gamma(K2SO4) of K2SO4 at m(K2SO4) in (KCl + K2SO4) mixtures have been determined at various constant values of total ionic strengths I of the mixed electrolyte studied. It has been found that log(gamma(KCl)) is a linear function of m(KCl) at each constant ionic strength studied, thus obeying Harned's rule. The corresponding trace activity coefficients, gamma((0)KCl), vary linearly with log I. Using Pitzer's scheme of equations has allowed fair reproduction of the experimental activity coefficients gamma(KCl), for I values greater than or equal to 0.5 mol.kg(-1) and the evaluation of gamma(K2SO4) in the same range.