The renularity of the variation in solvent extraction of Ianthanides (oxoept promethium) and yttrium by (ethylhexyl) isopropylphosphonate (DIOPP) from 3N and 7.4N HNO₃ of aqueous phase has been described in detail in this paper. The extraction equilibrium data at various temperatures (10~60℃) were determined. The regularity of the variation of these equilibrium data and thermodynamic funoLion was examined. The variation of water contents in organic phase was measured and showed tetrad effect roughly. The influence of the degree of nitrate complexing and hydration of the lanthanide ions on tho regularity of variation in the extraction process was discussed.