Abstract: Formation of third organic phase in PUREX or Thorex process is investigated under various conditions, because it may reduce the yield of Th(Ⅳ), U(Ⅳ) or Pu(Ⅳ), sometimes it may affect the normal operation of the process. In early period, the composition of the actinide(Ⅳ) tributyl phosphate solvate was assumed to be same in the unpartitioned organic phase and the formed third phase. But base on spectroscopy, the results indicate the presence of an extended solvate of M(Ⅳ). The effects of the temperature, the concentration of nitric acid and the extractant, the nature of diluent and the ionic strength of the aqueous phase on the formation of the third phase of M(Ⅳ) were investigated. Based on the mechanism studies, semi-empirical and empirical models of tetravalent actinide have been developed, correlating various third phase characteristics of third formation of Th(Ⅳ), U(Ⅳ) or Pu(Ⅳ). But these models have not yet to be reported used in the simulation of the process.