Abstract: It has been presented in this paper that Pr (Ⅳ) and Tb(Ⅳ) were prepared by oxidation of the corresponding trivalent species with ozone in aqueous ortho-tellurate solution. By means of ultraviolet and visible absorption spectroscopy, the existence of Pr(Ⅳ) and Tb(Ⅳ) in the solution has been proved. The characteristic absorption peaks for Pr(Ⅳ) are at 280, 447, 475 and 490nm, and that for Tb(Ⅳ) is at 360 nm. Kinetic studies on the solutions of Pr(Ⅳ) and Tb(Ⅳ) showed that the reductions of Pr(Ⅳ) and Tb(Ⅳ) in alkali aqueous solutions are pseudofirst-order reactions. The half-lives of Pr(Ⅳ) and Tb(Ⅳ) in these solutions are 18′14° and 2.65 days respectively. The potentials of the solutions which contained various concentrations of Tb(Ⅳ) in the presence of the complexing reagent have been measured and it has been found that under this condition the formal potential of the Tb(Ⅳ)/Tb (Ⅲ) couple is +0.953 V.