This paper reports the results obtained by studying the transport properties of novel highly stable activated composite membranes (ACM) containing di-2-ethylhexyl phosphoric acid as a carrier. ACM samples containing different amount of DEHPA have been prepared by varying the DEHPA concentration in the casting solution. The selectivity of Ho and Pr transport through ACMs has been shown to depend on pH of the feed solution and DEHPA concentration in the membrane. ACM samples with a relatively low carrier content exhibit a higher selectivity for Ho, while an increase of DEHPA concentration in ACM results in the reversal of membrane selectivities. The transport properties of ACM do not change within a week of continuous operation.