A suction line heat exchanger storage tank for use in a vapor compression system to increase the efficiency and capacity of the system. Carbon dioxide is preferably used as the refrigerant. The high pressure of the system (gas cooler pressure) is regulated by adding charge to or removing charge from the system and storing it in a storage tank. The suction line heat exchanger exchanges heat internally between the high pressure hot refrigerant fluid discharged from the gas cooler and the low pressure cool refrigerant vapor discharged from the evaporator. The high pressure is regulated by adjusting valves. A first valve allows excess charge from the system to enter the storage tank if the pressure in the gas cooler is too high. If the pressure in the gas cooler is too low, a second valve is opened to allow excess charge from the storage tank to reenter the system. By regulating the high pressure of the system, the evaporator inlet enthalpy can be controlled to achieve optimal efficiency and/or capacity.