The current paper investigates the performance behavior of an ORC unit in which the expansion process is realized by two in series scroll expanders. The heating load varies from 50 to 120 kWth and the heat supply temperature to the employed organic fluid (R245fa) from 80 to 130 °C. The two expanders in series configuration is chosen so that to achieve operation as close as possible to the nominal pressure ratio of each scroll machine at high evaporation temperatures where a total pressure ratio in the order of 10 is met. An extensive discussion and analysis is provided on how the key variables, such as pressure ratio, intermediate pressure and filling factor, interact between the two expanders and how this interaction affects each expander separately. This analysis leads to better understanding the overall performance in terms of isentropic and thermal efficiency and power generation. Emphasis is paid in off-design performance comprehension, while useful conclusions on how such split expansion process can be efficiently controlled by speed regulation are extracted. Focus is given on the interpretation of results for the partial heat load of 50 kWth at 130 °C, which show a very large scattering of both isentropic efficiencies of expanders and thermal efficiency. Accordingly, a variation of thermal efficiency is detected from slightly above 3% to about 10%. The results show that there are certain operating conditions where system performance is ameliorated and the maximum thermal efficiency of almost 10% is achieved, while the isentropic efficiency of high pressure expander reaches a maximum of 68% and that of low pressure expander 57%.