In alkaline solutions with NaOH concentration below 8 M, Pu(V) has been found to be unstable, disproportionating to form Pu(IV) hydrous oxide precipitate and dissolved Pu(VI). Sorption of Pu(V) on the freshly precipitating Pu(IV) hydrous oxide and partial reduction of Pu(VI) by water alpha-radiolysis products complicate disproportionation of Pu(V). Disproportionation and reproportionation equilibrium constants were measured in the range 4-8 M NaOH. The yield of disproportionation and reproportionation reaction products increase with temperature and sharply fall with NaOH concentration. (C) 1998 Elsevier Science S.A.