Abstract: Decomposition of tungsten ore concentrate is usually carried out by alkaline process and the intermediate product is crude sodium tungstate. It contains many impurities among which phosphorus is one of the most harmful elements. By using a solvent consisting of petroleum sulfoxide (PSO) and a diluent, the small amount of phosphorus can be removed almost completely in low acidic solution, i. e. at pH = 2.5 by countercurrent-multistage extraction. The extraction species is most likely H3PW12O40·PSO.