Abstract : The increase in emissions of greenhouse gases led the European Union to establish policies aiming at the reduction of such emissions. For mobile air conditioning systems, the EU decided to ban the use of refrigerants with GWP > 150 for the new platforms starting in 2011, and progressively for all new vehicles as of 2017. CO2 is a candidate refrigerant to the replacement of high GWP refrigerants. The thermodynamic properties of CO2 imply supercritical refrigerating cycle with low energy performance when the sink temperature is above 31°C. The high pressure of the system could be optimized in order to maximize the system COP. The exergy analysis of the cycle shows that the main irreversibilities are in the compression and isenthalpic expansion. The refrigeration cycle using two-phase ejector (equivalent to two-stage compression refrigeration cycle with total injection and isentropic expansion) is a solution to reduce irreversibilities. A 1D model has been elaborated to characterize the operation of the ejector and to design ejectors to be tested on a test bench. Comparative tests have been performed on CO2 refrigeration cycles with and without ejectors. The main test results are: -4 control strategies verified experimentally to regulate the high pressure and the evaporating temperature using the electronic expansion device and the compressor external control valve. -Validation of the nozzle model. -Validation of the 1D model by testing 18 different ejectors. The 1D model revealed that a gain in energy efficiency of about 12% is possible using the ejector in the CO2 cycle. The developed nozzles can be coupled to an impulse turbine wheel to become an isentropic expansion turbine. It could be interesting to use the ejector in an air refrigeration loop to evaluate its performances. The use of twophase ejector in CO2 heat pumps used to produce domestic hot water at 60°C leads to energy efficiency improvement of about 10% compared to CO2 classical systems.