Abstract: The analysis for Pr₆O₁₁ stored for ten years in laboratory at ambient environment shows that the sample contains about 19 wt% Pr (OH)₃. TG, FTIR and XRD analysis results show that there are three phases, namely Pr₆O₁₁, PrO₂ and Pr (OH)₃ in the sample. The mass change with the storage time was investigated in a closed ambient environment under the condition of 100 % relative humidity. The mass increasing tendency slows down after 60 d. Phosphate removal rate in aqueous solution was detected using pure Pr₆O₁₁ as adsorbent to find out that Pr₆O₁₁ reacts with H₂O to generate Pr (OH)₃. Pr₆O₁₁ can be completely converted into PrO₂ and Pr (OH)₃ under hydrothermal condition at 190 ℃. It is found that PrO₂ can also react with water slowly to generate Pr (OH)₃ at 190 ℃ for different hydrothermal time (24~96 h).