The thermal diffusivity of five platinum samples with resistivity ratios of 12, 34, 100, 900, and 5000 has been measured from 300° to 1200°K. Only the sample with the resistivity ratio of 12 showed deviations of more than 3% from the average thermal diffusivity of the two purest samples. Thermal conductivity values have been calculated from these results and compared with the results of other investigators. At room temperature most results are in good agreement, but at high temperatures the agreement among different investigators is unsatisfactory. The data from our purest samples indicate that platinum would be suitable for a thermal conductivity standard, but additional measurements should be made by other absolute methods.