A comprehensive thermodynamic model was developed to describe the dual liquid phase transition and vapor-liquid-liquid equilibria in biphasic solvent systems composed of aqueous diethylenetriamine and N,N,N',N '',N ''-pentamethyldiethylenetriamine for CO2 capture. The liquid- liquid phase separation with respect to volumetric fraction of either phase and partition of amines between the dual phases was predicted accurately. The model was also able to provide detailed speciation information for each liquid phase as a function of CO2 loading and temperature, and the predictions were consistent with the results from our prior nuclear magnetic resonance spectroscopic study. The predicted partial pressures of CO2 agreed with the experimental data; specifically, the model was used to predict the desorption pressures of CO2 and the heat of desorption reaction for the separated CO2 rich phase. The modeling approach developed in this study could be applied to other generic biphasic solvent systems.