The thermal performance of supercritical pressure ORCs (Organic Rankine Cycles) is related to the critical temperature of the organic fluids. The heat source in this investigation was flue gas with an inlet temperature of 150 degrees C and an outlet temperature of 70 degrees C. The working fluids were R218, R134a and R236fa. An integrated-average temperature difference was used to quantify the thermal match between the flue gas and the organic fluid in the evaporator. Three types of operating modes were identified: ()) a flexible operating mode for low T-c (critical temperature) fluids having operating states in a rectangular region in a plot of the turbine inlet pressures versus temperatures; (2) a bifurcated operating mode for moderate T-c fluids with one or two pressures corresponding to the turbine inlet temperature; (3) a restricted operating mode for high T-c fluids with only one turbine inlet pressure possible for the turbine inlet temperature. The high T-c organic fluid has a small integrated-average temperature difference that yields large evaporator and system exergy efficiencies. Thus, the useful power is increased. The low T-c organic fluid has a bad thermal match in the evaporator that leads to lower ORC (Organic Rankine Cycle) thermal performance. (C) 2013 Elsevier Ltd. All rights reserved.