Abstract: The extraction of uranium (Ⅳ) from hydrochloric acid medium by di-n-octyl sulphpxide (DOSO) in methylbenzene is studied and the stability of DOSO is determined. Results indicate that the distribution ratio of uranium (Ⅳ) increases with increasing hydrochloric acid concentration, and reaches a. maximum in the range of 9-10 M HC1 and then decreases. From the IR of DOSO, it can be seen that DOSO is unstable at high acidities, and begin to decompose at 8M HC1. The results confirm that the decrease of distribution ratio of uranium (Ⅳ) is due to the decomposition of DOSO by concentrated HC1 rather than competition between the metal and the acid for the available extractant. The species extracted appears to be UC14·3DOSO·HC1.