Abstract This paper presents the experimental characterization and modeling of a scroll expander. The expander used here is a scroll compressor modified to work as an expander. It is characterized in two experimental setups using air and ammonia as working fluids. The paper studies how the main operating variables (supply pressure and temperature, pressure ratio, rotational speed and lubrication) influence the performance of the scroll expander. A semi-empirical model is proposed to determine the scroll expander performance. This model uses some semi-empirical parameters (such as built-in volume ratio, leakage area and mechanical losses), obtained through experimentation, to calculate the mechanical power, supply mass flow rate and exhaust temperature. Using this semi-empirical model, the deviations in the calculated mechanical power, exhaust temperature and supply mass flow rate are ±9%, ±4 K and ±5% Hz.