Studies have been made on the extraction of some actinides [Th(IV), Pa(V), and U(VI)] and lanthanides [La(III), Ce(III), Pm(III), Eu(III), and Tb(III)] from hydrochloric acid medium by dialkyl sulfoxides. The actinides have extraction maxima at 7.0MHCl while the lanthanides practically do not get extracted. Thus the extraction separation of these two groups can be effected by the preferential extraction of the actinides by the sulfoxides.