A thermodynamic model of calculating mass action concentrations for structural units or ion couples in NaClO₄-H₂O and NaF-H₂O binary solutions and NaClO₄-NaF-H₂O ternary strong electrolyte aqueous solutions was developed based on the ion and molecule coexistence theory (IMCT). A transformation coefficient was needed to compare the calculated mass action concentration and the reported activity, because they were usually obtained at different standard states and concentration units. The results show that transformation coefficients between the calculated mass action concentrations and the reported activities of the same components change in a very narrow range. The transformed mass action concentrations of structural units or ion couples in NaClO₄-H₂O and NaF-H₂O binary solutions agree well with the reported activities. The transformed mass action concentrations of structural units or ion couples in NaClO₄-NaF-H₂O ternary solution are also in good agreement with the reported activities in a total ionic strength range from 0.1 to 0.9 mol/kg H₂O by the 0.1 mol/kg step with different ionic strength fractions of 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1, respectively. The results indicate that the developed thermodynamic model can reveal the structural characteristics of binary and ternary strong electrolyte aqueous solutions, and the calculated mass action concentrations of structural units or ion couples also strictly follow the mass action law.