This paper focuses on the experimental and numerical characterizations of a single-screw expander for waste heat recovery organic Rankine cycle (ORC) applications. A down-scaled industrial ORC test-rig has been tested with two different working fluids, R245fa and SES36. The hot source inlet temperature has been set to 125 degrees C and the maximum expander inlet pressure was limited to 1200 kPa. A total of 102 steady-state points have been collected by varying the expander pressure ratio between 3 and 9 with rotational speeds in the range from 2000 rpm to 3300 rpm. The experimental results allowed to assess the relationship between internal built-in volume ratio and imposed expansion ratio at different rotational speeds with respect to shaft and overall isentropic efficiency as well as volumetric performance in terms of filling factor. Results showed that while R245fa allowed approximately a 10% higher power output, the single-screw expander was performing at higher isentropic efficiency with SES36 due to higher pressure ratio achievable under the given working conditions and system limitations which also led to a better matching between ORC system and volumetric expander performance. A semi-empirical model has been developed and calibrated to break down the expander internal losses in the case of R245fa. The model has been exercised to investigate the effect of potential design improvements on the overall performance. The friction losses played a major role in the total loss count followed by suction pressure drops and leakages. As a consequence, the effect of lubrication should be further investigated to reduce leakages and friction. This study demonstrates the potential of single-screw technology as volumetric expander for ORC applications. (C) 2016 Elsevier Ltd. All rights reserved.