Vapor-liquid equilibrium, speciation, and amine solubility were studied for the piperazine (PZ)/methyldiethanolamine (MDEA)/water/carbon dioxide system. Solubility data for carbon dioxide in these solutions were obtained using a wetted-wall contactor. Solubility data for anhydrous PZ solid in water were obtained by adding PZ flakes to an agitated flask. In a 0.6 M PZ/4 M MDEA solution at 313 K, the partial pressure of carbon dioxide approaches one-tenth of the value in 4 M MDEA at low loading. This performance is between that of monoethanolamine (MEA) blends and diethanolamine (DEA) blends. The solubility data are modeled using the electrolyte NRTL model. The model is verified using C-13 and H-1 NMR data at high loading. The resulting model shows that the piperazine carbamate is the major reaction product at low loading (<0.1) while the protonated piperazine carbamate is the major reaction product at high loading. The piperazine dicarbamate species is present in higher concentration than in the simple PZ/H2O/CO2 system. Comparison to other blends (MEA/MDEA and DEA/MDEA) show that the dicarbamate and protonated carbamate species have a negative effect on the solubility Of CO2 at moderate loading. Amine volatility predictions are made using the Dortmund modification of the UNIFAC model to predict the activity coefficient of PZ in aqueous solutions. PZ volatility is very high; this model predicts PZ losses of the same magnitude as MDEA in a blend indicative of industrial concentrations, even though MDEA is present at much higher concentrations.