Recent studies indicate carbon dioxide (R744) as a valid alternative to classical substances such as HFCs used in vapour compression plants. However a transcritical refrigeration cycle is needed because the critical temperature of carbon dioxide is usually near the ambient temperature. Consequently the carbon dioxide refrigerator performances are significantly influenced by the heat rejection pressure. In this paper an experimental investigation on working optimization for a "split-system" to cool air in residential applications is presented: by varying the heat rejection pressure an optimum working condition has been found at different ambient temperatures. Furthermore a simplified model to predict the optimum heat rejection pressure is shown and a comparison with experimental results is carried out. Both the model validation and the experimental results Suggest that the heat rejection pressure optimization is an convenient method to improve the performance of a carbon dioxide split system. Finally an algorithm based on the aforementioned model has been proposed in order to control an electronic back pressure valve by means of a PLC. (C) 2009 Elsevier Ltd. All rights reserved.