An experimental and numerical investigation of the optimum ejector geometry for a small-capacity R744 heat pump was performed. Different ejector configurations were examined, including various lengths and diameters of the mixer and various angles of divergence for the diffuser. Based on a simplified, one-dimensional ejector model, an optimisation of the ejector geometry was performed. Based on both the numerical simulation and experimental work, the ejector efficiency proved to be notably dependent on the mixer length and the diameter, as well as on the diffuser divergence angle. The maximum increase in the coefficient of performance (COP) was 8% over a system with a conventional expansion valve.