This paper presents a study of an ejector expansion CO2 air conditioner system. A comprehensive analytical model for 3-ton air-to-air controllable ejector expansion transcritical CO2 air conditioners was developed and validated. The ejector component efficiencies were estimated using empirical correlations, instead of being simply assumed to be fixed values as done in other literature studies. Parametric studies of the ejector expansion CO2 systems show that the coefficient of performance (COP) and the cooling capacity reach maximum levels when the motive nozzle throat diameter becomes 2.8 mm; the maximum cooling COP and cooling capacity occurred for a mixing section constant-area diameter of between 4.1 and 4.2 mm; COP and cooling capacity are affected by the outdoor air temperature. At particular ejector geometries and an outdoor temperature of 37.8 degrees C, the transcritical CO2 system using an ejector as the expansion device outperformed a conventional expansion-valve transcritical CO2 system in COP and cooling capacity by approximately 30.7% and 32.1%. respectively. A careful selection of ejector geometry, such as motive nozzle throat diameter and mixing section constant-area diameter, is critical in order to improve the performance of an ejector expansion CO2 air conditioning system. (C) 2012 Elsevier B.V. All rights reserved.