Owing to the poor performance of the previous CO2 two-rolling piston expander prototype, improvements were made to increase its efficiency. The improvement measures included increasing the expansion ratio by minimizing the size of the intermediate channel and adopting some sealing methods to block the leakages, such as integrating the intermediate plate and filling O-rings in the seal grooves of the end covers. To reduce the leakage and irreversible losses, lubrication of the expander was investigated as a measure. The improved expander was tested in the CO2 water-to-water heat pump system. The experimental results showed that the revised expander had a much higher efficiency compared to the primary one, reaching a maximum of 77% at the revolution speed of 867 rpm, and a maximum recovered expansion work of 242 W at the revolution speed of 770 rpm. However, further improvement of the expander is necessary because the measured parameters of the prototype do not yet approach the design values. (C) 2015 Elsevier Ltd. All rights reserved.