Two-phase ejector studies often focus on the design and performance of the two-phase ejector and the effect it has on the performance of the ejector system. However, recent work has shown that evaporator design can also have a very significant effect on ejector cycle performance. In the present study, a numerical model of a microchannel air-to-refrigerant evaporator is developed and incorporated into thermodynamic models of the standard ejector and DX cycles. The models are used to investigate the effect of evaporator geometry, specifically the microchannel port hydraulic diameter, on the performance of the cycles. The analysis is performed using the refrigerants R134a and CO2, and the behavior of the two refrigerants with varying diameter is compared. The effects of evaporator outlet state and height-to-width ratio are also investigated. It is shown that evaporator design, not just ejector design, can have a significant effect on the COP of the ejector cycles.