In the present study, visualization and measurement of two-phase adiabatic expansion in a cylinder with moving piston are carried out. Experimental setup with piston and cylinder which mimics reciprocating expander was constructed and boiling phenomenon is visualized. The difference between measured and quasi-static pressures becomes larger as piston velocity is increased. With the decrease of piston diameter, agitation of the liquid phase becomes stronger and adiabatic efficiency becomes closer to that of the quasi-static state. The adiabatic efficiency is almost independent on the initial fluid temperature. Finally, a model to predict pressure change in two-phase expansion process is proposed and the model can predict adiabatic efficiency within the range of about +/- 2.5% accuracy. From the model prediction, it is shown that the uniformity of the liquid phase temperature distribution is the key factor for improving the adiabatic efficiency. (C) 2015 Elsevier Ltd. All rights reserved.