The paper focuses on operation of simple refrigeration cycles and considers the selection of controlled variables for two different cycles. One is a conventional sub-critical ammonia refrigeration cycle and the other is a trans-critical CO2 refrigeration cycle. There is no fundamental difference between the two cycles in terms of degrees of freedom and operation. However, in practical operation there are differences. For the ammonia cycle, there are several simple control structures that give self-optimizing control, that is, which achieve in practice close-to-optimal operation with a constant setpoint policy. For the CO, cycle on the other hand, a combination of measurements is necessary to achieve self-optimizing control. (c) 2007 Elsevier Ltd. All rights reserved.