A successful method has been developed for the determination of certain rare earths in thorium in the fractional ppm range. The procedure is based on the ionexchange chromatographic separation of the rare earths plus added yttrium carrier from the thorium, followed by emission spectrometric determination of the rare-earth impurities in the yttrium carrier. A simultaneous separation from the rare earths of the common element impurities present in the thorium has been accomplished. A nigh degree of compensation for procedural errors is achieved by the use of a pure rare earth as both the carrier in the separation and purification procedure and the matrix material in the spectrographic determination. The method fulfills the requirements of accuracy, sensitivity, and speed necessary for use in the routine analysis of reactor-grade thorium metal. It can find application, in its present form, also for the simultaneous determination of part-per-billion amounts of common element impurities in thorium. (auth).