Thin 99.99 pet purity molybdenum (Mo) foils for use in gas core reactors have been tested in both liquid and gaseous uranium tetrafluoride (UF4) at temperature ranges of 1500 to 2000 K under argon pressures approaching 600 torr for exposure times of 15 to 75 minutes. Weight change measurements have been performed before and after the experiments. Surface and cross-sectional analyses of the samples have been done using scanning electron microscopy (SEM), electron microprobe (EMP), and energy dispersive spectroscopy (EDS). The Mo container, which was exposed for nearly 9 hours to both liquid and gas phase UF4, was also examined. No reaction or diffusion of uranium or fluorine has been found on the cross sections of the samples under the experimental conditions. These results indicate that molybdenum can be a candidate wall material in contact with UF4 as used as a circulating fuel at ultrahigh temperatures.