With depleting fossil fuel reservoirs and the corresponding emissions of air pollutants, we urgently need to look for alternative, renewable and sustainable energy options to counteract our strong dependence on fossil fuels for energy supplies. Solar thermal, geothermal, biomass combustion heat energy and waste heat recovery may be applied to drive combined heat and power (CHP) systems with organic Rankine cycle (ORC). Working fluid is a critical factor to affect the efficiency of the ORC, therefore optimal selection of the working fluid for an ORC-based system needs to be studied. This paper presents the comparison and optimization of 8 mostly-applied working fluids nowadays and gives a preferable ranking by means of spinal point method. The organic working fluids (organic refrigerants) for the medium-to-low temperature ORC have thermodynamic, environmental and economic criteria, such as preferable boiling temperature, high enthalpy drop (i.e., high thermal efficiency), favourable heat transfer characteristics, highly thermal and chemical stability, non-flammability, low toxicity, no ozone depletion and low cost. According to these criteria and spinal point method, the 8 mostly-applied organic working fluids are characterized and listed in order of preferable selection: HFE7000, HFE7100, PF5050, R123, n-pentane, R245fa, R134a and isobutene. An adequate organic fluid should be selected considering these selection criteria and the specific heat source. (C) 2012 Elsevier Ltd. All rights reserved.