A vapor compressor system (200; 300; 400; 500; 600) comprising: a compressor (22; 220, 221); a heat rejection heat exchanger coupled to the compressor to receive refrigerant compressed by the compressor; an ejector having: a primary inlet; a secondary inlet; and an outlet; a heat absorption heat exchanger; a separator having: an inlet coupled to the outlet of the ejector to receive refrigerant from the ejector; a gas outlet; and a liquid outlet; and one or more valves (244, 246, 248, 250) positioned to allow switching of the system between: a first mode wherein: refrigerant passes from the heat rejection heat exchanger, through the ejector primary inlet, out the ejector outlet, to the separator; a first flow from the separator gas outlet passes through the compressor to the heat rejection heat exchanger; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger and through the ejector secondary port; and a second mode wherein: refrigerant passes from the heat rejection heat exchanger to the separator; a first flow from the separator gas outlet passes to the compressor; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger to the compressor characterized in that the system (200; 300; 400; 500; 600) further comprises a controller which is configured to switch between the first and second modes in response to sensed conditions comprising at least one of the outdoor ambient temperature and the compressor speed. a compressor (22; 220, 221); a heat rejection heat exchanger coupled to the compressor to receive refrigerant compressed by the compressor; an ejector having: a primary inlet; a secondary inlet; and an outlet; a primary inlet; a secondary inlet; and an outlet; a heat absorption heat exchanger; a separator having: an inlet coupled to the outlet of the ejector to receive refrigerant from the ejector; a gas outlet; and a liquid outlet; and an inlet coupled to the outlet of the ejector to receive refrigerant from the ejector; a gas outlet; and a liquid outlet; and one or more valves (244, 246, 248, 250) positioned to allow switching of the system between: a first mode wherein: refrigerant passes from the heat rejection heat exchanger, through the ejector primary inlet, out the ejector outlet, to the separator; a first flow from the separator gas outlet passes through the compressor to the heat rejection heat exchanger; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger and through the ejector secondary port; and a second mode wherein: refrigerant passes from the heat rejection heat exchanger to the separator; a first flow from the separator gas outlet passes to the compressor; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger to the compressor a first mode wherein: refrigerant passes from the heat rejection heat exchanger, through the ejector primary inlet, out the ejector outlet, to the separator; a first flow from the separator gas outlet passes through the compressor to the heat rejection heat exchanger; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger and through the ejector secondary port; and refrigerant passes from the heat rejection heat exchanger, through the ejector primary inlet, out the ejector outlet, to the separator; a first flow from the separator gas outlet passes through the compressor to the heat rejection heat exchanger; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger and through the ejector secondary port; and a second mode wherein: refrigerant passes from the heat rejection heat exchanger to the separator; a first flow from the separator gas outlet passes to the compressor; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger to the compressor refrigerant passes from the heat rejection heat exchanger to the separator; a first flow from the separator gas outlet passes to the compressor; and a second flow from the separator liquid outlet passes through the heat absorption heat exchanger to the compressor characterized in that the system (200; 300; 400; 500; 600) further comprises a controller which is configured to switch between the first and second modes in response to sensed conditions comprising at least one of the outdoor ambient temperature and the compressor speed.