Abstract: Although quite a number of tetravalent praseodymium compounds have been prepared and well characterized,the solution chemistry of tetravalent praseodymium is little known.By means of "Carrier oxidation"through cerium from a 4:1 mixture of Ce (Ⅲ) and Pr (Ⅲ) nitrate in aqueous solution,using ozone as oxidizing agent at room temperature and at pHaround 6.5,a more reactive heteropolynuclear oxide hydrate with chocolate brown color was obtained.After drying in vacuo at a temperature not higher than 50℃,chemical analysis and X-ray diffraction pattern( Fig,1,2).confirmed the formation of PrO₂in the mixed oxide.The degree of readiness for that heteropolynuclear oxide to get into solution differs markedly in con.HCl,con.HNO₃and glacial acetic acid.The stability in those solutions also appearsdifferent.In con.HCl,solution formation is very easy;the blood red color due to Pr(Ⅳ) lasts for only a few minutes.Solution formation in con.HNO₃is less easy,but the blood red color lasts much longer.In nitric acid solution,oxidation of MnO₂·_.H₂O to MnO₄^- has been observed visually;transient formation of H₂O₂has been revealed in recording spectrophotometer(Fig.4,7).These phenomena substantiate the relative values of their respective electrode potentials given in literature.Solution of Ce(Ⅳ)-Pr(Ⅳ) heteropolynuclear oxide into glacial acetic acid needs brief warming;the blood red color can persist for a long time.Mixed complex compounds containing Ce(Ⅳ) and Pr(Ⅳ) can be obtained.