Abstract: The stability of complex compounds formed by glycyl glycine, leucine and 2,3,5,6-tetrahydro-6-phenylimidazo (2,1-6) thiazole with uranyl ion (II) has been investigated. The stepwise formation constants of these compounds were determined potentiometrically at 25℃ in aqueous solution in presence of 0.1 Mpotassium chloride.The adsorption theory of complex equilibria in solution which has been proposed by Hsu Kwang-hsien[4] , was introduced in treatment of the data determined by pH method.