Aqueous ammonia is an attractive absorbent for the capture of CO₂ from industrial flue gases. However evaporative ammonia loss limits the application of the ammonia-based CO₂ capture process. The use of metal ion additives (where M(II) = Cu(II), Zn(II) and Ni(II)), has been reported previously for the effective suppression of ammonia loss in the post combustion capture of CO₂. In this study, we have investigated the absorption of CO₂ into various M(II)/NH₃ solutions, the resulting loss of ammonia to evaporation, and have attempted to rationalise the observed effects using an extended chemical model that includes all chemical reactions in solution. The theoretical and experimental investigations suggest that complexation of ammonia by metal ions effectively reduces the concentration of free ammonia, resulting in reduced evaporative loss but also reduced reactivity towards CO₂. Simple reduction of the total NH₃ concentration to the same concentration of free NH₃ concentration as that obtained by the addition of M(II) showed a similar reduction in the ammonia loss as well as the CO₂ absorption rate. The effects on CO₂ absorption capacity at 25 and 80 °C, cyclic capacity and regeneration energy are also simulated and discussed.