The nature of the complex europium carbonate ion in concentrated carbonate solution has been studied polarographically and its existence has been proved by means of anion-exchange resins in the carbonate form 1. In the present work, the effects of pH and carbonate ion concentration on the reduction wave and electrode reaction were studied in detail. The electrode reaction was always irreversible and consisted of a one-electron change. From the slope of d(E&/d log(CQa+) (or d(El&/d log 4, the coordination number for carbonate was found to be one ; the existence of Eu(COs) + ion at a PH above 10.0 was presumed from the electrode reaction and europium(I1) carbonate as the basic salt was assumed to be precipitated on the mercury electrode after reduction. On this basis, a polarographic method for the determination of micro amounts of europium ion in the presence of other diverse ions and rare earth ions was developed. Europium could be determined even in presence of the complex cerium carbonate ion. Under the optimum conditions europium in the range 2.10-4 M to 2.10-3 M could be determined successfully in mixtures of rare earth oxides. A@aratacs EXPERIMENTAL Yanagimoto's photographic polarograph PEL-model 3 Type and a pen recorder were used to record the current-voltage curves. All cathode potentials were corrected and referred to the saturated calomel electrode (S.C.E.) prepared by the method of HUIW AND HARRIS~. The sensitivity of the automatic recorder was 0.04 PA/mm. The capillary used had an wz value of 0.680 mg/sec and a drop-time of 3.20 set at the potential of -1.83 V vs. S.C.E. when measured in an oxygen-free 1.5 M carbonate solution at an effective height of mercury of 67.61 cm. The value of the capillary constant rn2/Wlo was about o-938. The PH was measured with a Hitachi glass electrode pH meter model EHI\I-I. All experiments were carried out in a thermostat at 25.0 & 0.1~. The value of the half-wave potential was obtained by wave analysis of logarithmic plots, and that of the diffusion current by compensation of residual current. Che7uicals Europium stock solution was prepared by dissolving pure europium perchlorate made from pure oside (purity more than gg.ggO/o), in distilled water and was stan-Amtl. Chiw. A&c, 28 (x+3)