Natural refrigerants are recognised as the most promising working fluids to replace conventional HFCs in food refrigeration systems. Due to its negligible GWP, zero ODP and attractive thermo-physical properties, the CO2 working fluid has grown in popularity over the last decade, especially in supermarket refrigeration systems. These systems can be classified into three main categories: indirect, cascade and all CO2 transcritical booster structures. However, the appropriate control and design optimisation of such systems require further investigation. This paper presents an experimental investigation into the performance of CO2 finned-tube gas coolers/condensers with different designs in a CO2 booster system. The heat exchangers were mounted in a specially designed test facility that allowed the control of different test conditions and parameters, including air on temperatures and flow rates, approach temperatures and CO2 operation pressures. The integrated refrigeration system can provide specified CO2 fluid parameters at the heat exchanger inlet, through which the system efficiency can be calculated. Subsequently, extensive measurements were recorded from this test rig, with insightful indications into system performance and the most influential parameters for system optimisation. These include heat exchanger designs, air on temperatures and flow rates, supercritical and subcritical pressure controls and cooling capacity controls, which are described in the paper. (C) 2016 Elsevier Ltd. All rights reserved.