This article presents an exergetic analysis for an organic Rankine cycle that converts waste energy to power from medium-grade heat sources (503 to 923 K). In addition, the effect of the waste heat temperature, the evaporator pressure, and the pinch point temperature difference on the thermal and exergetic performance of the system is studied. Several organic working fluids were selected to investigate the effect of the fluid's critical temperature on the performance of organic Rankine cycles. The working fluids under investigation are R245fa, R123, R142b, Isobutane, R113, and R141b with critical temperatures in the range of 407.7 to 524.9 K.