Fe(III) ions (concn. 0.10 M) were sepd. from an acidic aq. soln. (pH 0.3) contg. also Mn(II), Cu(II), Co(II), Ni(II), Na and Cl ions by extn. through a cellulose triacetate inclusion membranes with [Me(CH(2))(7)](3)PO (TOPO) as an ion carrier and o-nitrophenyloctyl ether as plasticizer. Distd. water or an aq. HCl soln. were used as receiving phase. The Fe(III) sepn. selectivity coeff. increased with increasing content of TOPO in the membrane up to 426.2 (Fe/Mn), 639.3 (Fe/Ni), 127.9 (Fe/Cu) and 319.7 (Fe/Co) at the TOPO conc. 0.25 M.