Twenty-two materials were screened as candidates for high temperature thermophysical property reference standards. Of these five (alumina, thoria, tungsten and two graphites) were selected for an initial field test program involving six laboratories in the U.S. Thermal conductivity and thermal diffusivity data and physical-chemical stability results on these materials in the temperature range from 500 to 2700°K are presented. Two of the materials, tungsten and an isotropic graphite, were selected for more detailed evaluation in an expanded international measurement program which involved 18 laboratories, 9 in Europe and 9 in the U.S. It was concluded with some qualifications which are described that the thermal conductivity and thermal diffusivity of the graphite were known within ± 10 per cent and ± 6 per cent respectively while for the tungsten the corresponding inaccuracy bound for both properties was ± 5 per cent. Although greater data scatter was observed among the tungsten results, the uncertainty estimates in the thermophysical properties were reduced using high precision electrical resistivity data. The experimental results on graphite and tungsten were found to be consistent based on theoretical considerations. Specific recommendations are given for further experimental and analytical studies with these two calibration materials.