The use of (GO) 2PO(OH) and (GO)PO(OH) 2 as extractants in a carrier solvent against an opposing aqueous mineral acid phase in effecting certain mutual separations in the actinide (III) series and lanthanide (III) series, including Y(III), has been investigated. The separation of Cf (and E and Fm) from Cm (and Am) by use of a di 2-ethylhexyl phosphoric acid-HCl system and the separation of Cf (and E and Fm) from Y by use of a diphenyl phosphoric acid-HCl system have been shown to be applicable to nuclear problems on a laboratory scale. It seems likely that these systems could be applied even more efficiently to large-scale operations. The separation of Bk(III) from Ce(III) by use of a di-2-ethylhexyl phosphoric acid-HCl system and the separation of Am(III) from Pm(III) by use of a mono 2-ethylhexyl phosphoric acid-HCl system have also been demonstrated. Other systems suitable for the latter separation are reported also. No system readily applicable to the separation of Am and Cm has been developed. The depressant effect of an alcohol on the extractive properties of (GO)PO(OH) 2 is noted, and the corresponding depressant effect of (GO) 3PO on the extractive properties of (GO)PO(OH) 2 is utilized in preventing interference by the latter ester in certain separations dependent upon the extractive actions of (GO) 2PO(OH).