The extraction of UO2+2 with the extractant alpha,omega-bis-(1'-phenyl-3'-methyl-5'-hydroxy-pyrazol-4'-oyl) alkane, 'HL-n-LH' with n, number of methylene -CH2- links, HL-4-LH (with a short polymethylene chain), has been described for a wide range of solution concentration at acid pH and compared to its extraction with HL-8-LH (with a long polymethylene chain). The efficient extraction of uranium (10(-3) M) from its aqueous acid solution was prevented by the appearance of a third phase and loss of uranium with the shorter chain length HL-4-LH. The results demonstrated a non quantitative distribution which is not found with HL-8-LH, in the same experimental conditions. The aim of this work is to make a contribution by trying to solve the third phase problem. The very low detection limits of energy dispersive X-ray fluorescence (EDXRF) allowed dilutions, by using the concentration range 4 x 10(-3)-2 x 10(-2) M for HL-4-LH in CHCl3 as organic phase and [UO22+] = 10(-4) M from perchlorate medium (Na, H)+ClO4 1 M as aqueous phase and the titration of aqueous and organic extraction phases, for the systems UO22+, (Na, H)+ClO4- 1 M, aqueous/HL-4-LH. diluent, was performed adequately. The extraction efficiency of uranium for HL-8-LH was better across the concentration range 0.5 x 10(-2)-4 x 10(-2) M at low pH than for HL-4-LH. The extracted complexes were UO2(L-4-LH)(2), [UO2(L-4-L)](2). UO2(L-8-L), UO2(L-8-LH)(2), [UO2(L-8-L)](2). (C) 2003 Elsevier Science B.V. All rights reserved.