Abstract: In this paper, a azo-type color forming reagent——p-Acetylarsenazohas been developed for an eluting agent. It has been used for the cation exchange separation of thorium and rare earths (uranium, scandium). The eluting components can be determinated directly with spectro-photometric technique. The method is more simple and rapid. The separation and continuous determination of the rare earths and thorium in the samples of ores were carried out. The results are satisfactory.