This paper describes the results of a series of experiments that were performed to define the performance of the uniflow piston-operated valve steam engine developed by the Australian National University (ANU) for solar thermal applications. The results are fitted by least-squares regression to simple power series expressions that correlate the power output and engine inlet pressure to steam temperature, thermal power input and condenser pressure. The performance of the engine is examined by comparison with an idealized frictionless and quasi-static model. The idealized model is used to assess the relative importance of the effects of friction and non equilibrium thermodynamic conditions in the engine. The exergetic performance of the ideal engine is examined to demonstrate the fundamental limitations of this engine technology. It is concluded that high piston/cylinder forces during the first stages of expansion pose a significant barrier to the enhancement of engine efficiency through the use of higher temperatures and expansion ratios.