Abstract: The calculating models of mass action concentration for electrolyte aqueous chloride solutions KBr-H₂O, NH₄Br-H₂O, and ZnBr₂-H₂O were founded at 298.15 K to calculate the molalities before saturation according to the ion and molecule coexistence theory and the mass action law. The calculated mass action concentration is based on pure species as the standard state and mole fraction as the concentration unit, and the reported activities are usually based on infinite dilution as the standard state and molality as the concentration unit. Hence, the calculated mass action concentration must be transformed to the same standard state and concentration unit. The transformation coefficients between calculated mass action concentrations and the reported activities of the same component fluctuate in a very narrow range. Thus, the transformed mass action concentrations not only agree well with the reported activities, but also strictly obey the mass action law. It is indicated that the new developed models can embody the intrinsic structure of the investigated three electrolyte aqueous solutions. Electrolyte aqueous solutions have characters of ideal solution under the assumed condition. The results also show that the mass action law has its widespread applicability to electrolyte binary aqueous solutions.