A refrigerant vapor compression system comprising: a first compression device having a refrigerant discharge outlet and a refrigerant suction inlet, a refrigerant heat rejection heat exchanger, a primary expansion device, and a refrigerant heat absorption heat exchanger; a refrigerant-to-refrigerant heat exchanger having a first refrigerant flow pass, a second refrigerant flow pass and a third refrigerant flow pass, said first refrigerant flow pass interdisposed in a primary refrigerant circuit between a refrigerant outlet of said refrigerant heat rejection heat exchanger and a refrigerant inlet to said refrigerant heat absorption heat exchanger; said second refrigerant flow pass disposed in heat exchange relationship with at least said first refrigerant flow pass and said third refrigerant flow pass; and an economizer refrigerant circuit including an economizer refrigerant line having an inlet in refrigerant flow communication with the primary refrigerant circuit at a location downstream of the refrigerant outlet of said refrigerant heat rejection heat exchanger and upstream of the refrigerant inlet to said refrigerant heat absorption heat exchanger, said second refrigerant flow pass interdisposed in said economizer refrigerant circuit; characterised in that the system further comprises a second compression device connected in serial refrigerant flow communication with said first compression device in the primary refrigerant circuit, said second compression device having a refrigerant discharge outlet and a refrigerant suction inlet, the refrigerant discharge outlet of said second compression device being connected in refrigerant flow communication with the refrigerant suction inlet of said first compression device; in that said third refrigerant flow pass is interdisposed in the primary refrigerant circuit between the refrigerant discharge outlet of said second compression device and the refrigerant suction inlet of said first compression device; and in that the economizer refrigerant line of said economizer circuit is in refrigerant flow communication with an intermediate pressure stage of said second compression device. a first compression device having a refrigerant discharge outlet and a refrigerant suction inlet, a refrigerant heat rejection heat exchanger, a primary expansion device, and a refrigerant heat absorption heat exchanger; a refrigerant-to-refrigerant heat exchanger having a first refrigerant flow pass, a second refrigerant flow pass and a third refrigerant flow pass, said first refrigerant flow pass interdisposed in a primary refrigerant circuit between a refrigerant outlet of said refrigerant heat rejection heat exchanger and a refrigerant inlet to said refrigerant heat absorption heat exchanger; said second refrigerant flow pass disposed in heat exchange relationship with at least said first refrigerant flow pass and said third refrigerant flow pass; and an economizer refrigerant circuit including an economizer refrigerant line having an inlet in refrigerant flow communication with the primary refrigerant circuit at a location downstream of the refrigerant outlet of said refrigerant heat rejection heat exchanger and upstream of the refrigerant inlet to said refrigerant heat absorption heat exchanger, said second refrigerant flow pass interdisposed in said economizer refrigerant circuit; characterised in that the system further comprises a second compression device connected in serial refrigerant flow communication with said first compression device in the primary refrigerant circuit, said second compression device having a refrigerant discharge outlet and a refrigerant suction inlet, the refrigerant discharge outlet of said second compression device being connected in refrigerant flow communication with the refrigerant suction inlet of said first compression device; in that said third refrigerant flow pass is interdisposed in the primary refrigerant circuit between the refrigerant discharge outlet of said second compression device and the refrigerant suction inlet of said first compression device; and in that the economizer refrigerant line of said economizer circuit is in refrigerant flow communication with an intermediate pressure stage of said second compression device.