A carbon dioxide refrigerant vapor compression system comprising: a refrigerant circuit including a refrigerant compression device, a refrigerant heat rejection heat exchanger for passing carbon dioxide refrigerant received from said compression device at a high pressure in heat exchange relationship with a cooling medium, a refrigerant heat absorption heat exchanger for passing carbon dioxide refrigerant at a low pressure refrigerant in heat exchange relationship with a heating medium, and an expansion device disposed in the refrigerant circuit downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heat absorption heat exchanger; and a flash tank receiver disposed in the refrigerant circuit downstream of said refrigerant heat rejection heat exchanger and upstream of said main expansion device; characterised by further comprising a compressor unload circuit including a refrigerant line establishing refrigerant flow communication between an intermediate pressure stage of said compression device and said refrigerant circuit at a location downstream of said refrigerant heat absorption heat exchanger and upstream of a suction inlet to said compression device, and a unload circuit flow control device disposed in said unload circuit refrigerant line; and a controller operatively associated with said expansion device and said unload circuit flow control valve, said controller operative in response to at least one system operating parameter sensed by at least one sensor (105,106) to selectively position the unload flow control device to maintain said refrigerant vapor compression system operating at a discharge pressure below a preselected upper limit; wherein the sensed operating characteristic is refrigerant discharge pressure or refrigerant discharge temperature. a refrigerant circuit including a refrigerant compression device, a refrigerant heat rejection heat exchanger for passing carbon dioxide refrigerant received from said compression device at a high pressure in heat exchange relationship with a cooling medium, a refrigerant heat absorption heat exchanger for passing carbon dioxide refrigerant at a low pressure refrigerant in heat exchange relationship with a heating medium, and an expansion device disposed in the refrigerant circuit downstream of said refrigerant heat rejection heat exchanger and upstream of said refrigerant heat absorption heat exchanger; and a flash tank receiver disposed in the refrigerant circuit downstream of said refrigerant heat rejection heat exchanger and upstream of said main expansion device; characterised by further comprising a compressor unload circuit including a refrigerant line establishing refrigerant flow communication between an intermediate pressure stage of said compression device and said refrigerant circuit at a location downstream of said refrigerant heat absorption heat exchanger and upstream of a suction inlet to said compression device, and a unload circuit flow control device disposed in said unload circuit refrigerant line; and a controller operatively associated with said expansion device and said unload circuit flow control valve, said controller operative in response to at least one system operating parameter sensed by at least one sensor (105,106) to selectively position the unload flow control device to maintain said refrigerant vapor compression system operating at a discharge pressure below a preselected upper limit; wherein the sensed operating characteristic is refrigerant discharge pressure or refrigerant discharge temperature.