Abstract: A potentiometric study of the complex formation between citric acid (H3L) and the uranyl ion at 25° and ionic strengths of 0.1 and 1.0 (KNO3) is reported From the concentration dependence of the formation constant, it is concluded that polynuclear complexes are formed in which bridging between metal ions occurs through carboxylate and hyroxyl groups of the ligand. The values of the logarithms of the formation constant of the metal checlate [UO2L-]/[UO2+][L3-] and of the dimerization constant [(UO2)xL22-]/[UO2+][L3-] are found to be 7.40 and 4.07 respectively. Infrared absorption measurements of protonated and dissociated carboxyl groups in aqueous uranyl citrte system indicated the presence of both carobxy-late and hydroxide bridging in the polynuclear complex. On the basis of the "core plus links" treatment of polynuclear complexes, the polymeric species in solution in the buffer region between 3 and 4 2/3 moles of base per mole of metal complex appears to be predominantly (U)2)2L2((OH)5(UO2)2L2)216-.