Thermodynamic models of calculating mass action concentrations of structural units or ion couples in RbCl-H2O binary and RbCl-RbNO3-H2O ternary strong electrolyte aqueous solutions has been developed based on the ion and molecule coexistence theory at 298.15K.A transformation coefficient is needed to compare calculated mass action concentration and reported activity because they are obtained at different standard state and concentration unit.The results show that transformation coefficients between calculated mass action concentrations and reported activities of the same structural units or ion couples change in a very narrow range.The transformed mass action concentrations of structural units or ion couples in RbCl-H2O binary system are in good agreement with that of reported activities.