CsF + CsBr + H2O and CsF + CsNO3 + H2O mixed-electrolyte systems were investigated by the potentiometric method. The experimental data were modeled by connecting the Nernst equation with the Pitzer or Harped model. The mean ionic activity coefficients of CsF, gamma(+/- A) and CsBr or CsNO3, gamma(+/- B), were calculated. Moreover, other properties, such as mean ionic activity coefficients in pure and trace quantities (gamma(0) and gamma(tr)), osmotic coefficients, Phi, excess Gibbs energies, G(E), and the change in excess Gibbs energy of the mixture, Delta G(m)(ex), were obtained and discussed.