BERYLLIUM hydroxide and also the precipitates obtained by the addition of alkali carbonates, bicarbonates or ammonium carbonate to the solutions of beryllium salts have been known to be soluble in excess of the above precipitants. (1) As beryllium remaining in solution in the above reagents does not respond to its usual qualitative tests, it can be presumed that beryllium remains in such solutions as a complex carbonate. DEBRAY (2) claimed to have isolated compounds of the formulae 3K2Be(CO3)2.Be(OH)2 and 3(NH4)2Be(CO3)2.Be(OH)z from the solutions obtained by dissolving beryllium hydroxide in K2COs and (NH4)2CO3 respectively. ATTERBERG, (3) however, regarded the two compounds to be mixtures, whilst HUMPIDGE (4) assigned the formula 2(NH4)2(CO3)~.Be(OH),o to the ammonium compound. During the course of the present work, (5) we came across several communications by PIRTEA and coworkers, (~} who reported a gravimetric method on the isolation of [Co(NH3)6][(HzO),~Be,,(CO3),,(OH)3],3H20. These works, however, do not give us a clear idea regarding the actual composition of berylli-carbonato complex either in solution or in the solid state, and this consideration has led the present author to undertake a study of the formation of beryllium carbonato-complexes by physico-chemical methods and their isolation. From transport and ion-exchange experiments the complex berylli-carbonate ion has been found to be anionic in nature. But no definite information regarding the reacting ratio of beryllium and bicarbonate ions leading to the formation of complex berylli-carbonate ions could be available from the conductometric and thermometric titration data obtained by the reaction between beryllium and bicarbonate ions. The estimation of CO 2 evolved during the reaction between beryllium and excess bicarbonate ions, after subtraction of the amount of CO 2 determined in the blank experiment, however, indicated that 4 moles Be evolve 6.9 moles CO x. This result, which is susceptible to give not very accurate information because of the evolution of large amount of CO x in the blank experiment, suggests the probable occurrence of either of the first two reactions below (vide E X P E R I M E N T A L ) , but not of the third one.