A Rankine cycle using organic fluids as working fluids, called organic Rankine cycle (ORC), is potentially feasible in recovering low enthalpy containing heat sources. Efficient operation of the ORC depends heavily on two factors: working conditions of the cycle and the thermodynamic properties of the working fluids. The working fluids under investigation are Benzene (C6H6), Toluene (C7H8), p-Xylene (C8H10), R113 and R123. Among the working fluids under investigation, p-Xylene shows the highest efficiency while Benzene shows the lowest. The study also shows that the irreversibility depends on the type of heat source. Generally speaking, p-Xylene has the lowest irreversibility in recovering a high temperature waste heat, while R113 and R123 have a better performance in recovering a low temperature waste heat. (C) 2000 Elsevier Science Ltd. All rights reserved.