Trace quantities of thorium in uranium were separated by precipitating thorium fluoride from a 1.0M HF-1.0M NH/sub 4/F matrix using yttrium as an internal standard and lanthanum as a carrier. The thorium precipitate was collected on a filter and analyzed by x-ray emission spectrometry. The method has a detection limit of about 15 mu g thorium per gram of uranium. The relative accuracy and precision at the 120- mu g thorium level was 95.5 plus or minus 1.5%. Studies on the rate of precipitation, sample size limitations, and the effect of varying molar fluoride-touranium ratios are discussed. The effects of potentially interfering anions, cations, and acids, which are commonly found in uranium system or form insoluble fluorides were studied. Radioactive tracer studies using /sup 234/Thh and /sup 88/Y were carried out at different levels of thorium and uranium to determine the absolute efficiency of the precipitation. The speed, selectivity, and precision of this x-ray method make it an excellent alternative to spectrophotometric methods. (auth)