Abstract. The distribution coefficient of U(VI) between perchloric acid solutions and benzene solutions of TTA has been measured as a function of the acidity. It was possible to interpret the data in terms of the charge on the U(VI) species present in the aqueous phase. The experiments indicate that U(VI) exists as an ion charge +2 at acidities from 0.1M to 2.5M. In the interpretations of the distribution data, it was necessary to know the acidity coefficient of the uranyl chelate in the benzene solution. This was determined by measurement of the solubility of the compound UO2K2 - xH2). The activity coefficient of UO2K2 was found to decrease rapidly as the concentration of TTA in the benzene phase increased above 0.1M. This is probably due to the formation of a species involving more than two TTA molecules per U(VI)