The present invention provides an improved, commercially available organic working fluid, which is operable under a broad range of temperatures, is thermally stable, has a high auto-ignition temperature, low freezing point and high critical temperature and is benign to the environment, and safe for human use. Such an organic working fluid is useful in organic Rankine cycle (ORC) power plants or units and other systems of the like; as an intermediate fluid for heat-recovery wherein heat from various heat sources is transferred using the intermediate fluid to a further working fluid and converted into work, and the intermediate fluid is also exploited to produce electricity. Such organic working fluids are also operable as heat transfer fluids either in ORC power plants or units or in other heat transfer systems. For this purpose the present invention presents a working fluid comprising at least one highly branched, heavy iso-paraffin hydrocarbons, or a mixture of two or more of such hydrocarbons. Preferably, at least one highly branched iso-paraffin hydrocarbon is present as the major component (i.e. at least 50% by volume) in the working fluid. A preferred sub-class of the class of the branched iso-paraffins which are suitable to be incorporated in organic working fluids of the present invention includes 8 to 20 carbon atom-containing hydrocarbons having at least one methyl radical (CH.sub.3) arranged to achieve a highly stable compound.