The flexibility and applicability of the modified Huron-Vidal second order (MHV2) mixing rule have been examined for the correlation of solubilities for CO2 and NH3 in H2O as an example of a system containing volatile weak electrolytes. The MHV2 mixing rule is one of the excess Gibbs energy (g(E)) mixing rules. The g(E) mixing rules are well known to be able to predict vapor-liquid equilibria (VLE) of polar or non-polar mixture systems under high pressure. We have used to Soave-Redlich-Kwong equation of state (SRK-EOS) with the MHV2 mixing rule. Assuming a series reaction between CO2 and NH3 in the aqueous phase and the temperature dependence of the equilibrium constant, the solubility behavior of the CO2-NH3-H2O system can be correlated with reasonable accuracy.