Abstract: 8TUDY ON THE HYDROLYSIS REACTION OF NP（Ⅴ）LI YADONG；LI GUODING（Department of Environmental Engineering，Tsinghua University, Beijing，1 00084）ABSTRACT Hydrolysis reaction of Np（Ⅴ）in near neutral solution is studied by spectrophotometry.The absorption spectra of Np（Ⅴ）at various pH are obtained．The first hydrolysis constantof Np（V）is determined．The second hydrolysis reaction is confirmed to occur whenpH＞10，producing a negatively charged species. Key words Neptunium Hydrolysis Absorption spectrum