In the temperature range of 450–700 K, there are currently no working fluids that have been validated for heat pipes and loop heat pipes, with the exception of water in the lower portion of the range. This paper reviews a number of potential working fluid including several organic fluids, mercury, sulfur/iodine, and halides. Physical property data are used where available, and estimated where unavailable using standard methods. The halide salts appear to possess attractive properties, with good liquid transport factors, and suitable vapor pressures. Where nuclear radiation is not a consideration, other potential working fluids are aniline, naphthalene, toluene, and phenol. The limited available life test data available suggests that toluene, naphthalene, and some of the halides are compatible with stainless steel, while the other fluids have not been tested. © 2004 American Institute of Physics