Thermodynamic analyses and comparison of ammonia-, propane- and isobutane-based vapour compression refrigeration cycles are presented in this article using constant area mixing ejector as an expansion device. Optimization of ejector geometric parameter based on the maximum cooling COP and performance improvement for different operating conditions is studied. Results show that the optimum geometric parameter increases with increase in evaporator temperature and decrease in condenser temperature, whereas the COP improvement over basic expansion cycle increases with the increase in condenser temperature and decrease in evaporator temperature. Expressions for optimum ejector geometric parameters have been developed, which offer useful guidelines for optimal design and operation. Study shows that the optimum parameters as well as COP improvement using ejector as expansion device are strongly dependent on the refrigerant properties as well as the operating conditions. The optimum geometric parameter is maximum for ammonia, whereas minimum for isobutane. Using ejector as an expansion device, isobutane yields maximum COP improvement of 21.6% followed by propane (17.9%) and ammonia (11.9%) for studies ranges. Copyright © 2009 John Wiley & Sons, Ltd.