Piperazine (PZ) has been reported as an effective rate promoter in the aqueous ammonia-based solvent process for the post combustion capture (PCC) of CO₂. However, the detailed promotion effect of PZ on CO₂ absorption into partially loaded ammonia solutions and the mechanism of this process are still unclear. In an effort to determine the detailed promotion effect of PZ in aqueous ammonia-based solvents, overall mass transfer coefficients (K_G) describing the absorption of CO₂ into aqueous PZ/NH₃ solutions were determined using a wetted-wall column apparatus at 25 °C. The effect of added PZ (from 0 to 0.5 M) on the mass transfer of CO₂ into 3.0 M NH₃ solutions over a range of pre-loaded CO₂ concentrations of 0.9 M at 25 °C are reported in this work. The fast kinetic reactions of CO₂(aq) with blended solutions containing PZ/NH₃ were investigated using stopped-flow spectrophotometry at 25.0 °C. Analysis of the kinetic measurements using a chemical model which incorporates the complete reaction sets of the individual amines with CO₂ (i.e., NH₃-CO₂-H₂O and PZ-CO₂-H₂O) resulted in good agreement with the experimental data. The contribution distribution from each reactive species was calculated based on the proposed reaction scheme of the PZ-NH₃-CO₂-H₂O system. Results show that both the PZ/PZH⁺ and PZCO2− /PZCO₂H pathways make contributions to the promotion of CO₂ absorption into PZ promoted aqueous NH₃ solutions. Importantly, the reactive piperazine mono-carbamate species, PZCO2− /PZCO₂H, which is present in the CO₂-loaded mixtures of PZ/NH₃, plays an important role in the promotion of CO₂ absorption into CO₂-loaded aqueous NH₃ solutions. The mass transfer simulation results reveal that there are additional reactions occurring in the gas–liquid interface and gas phase due to the volatility of NH₃, which requires further improvement on the simulation model.