This research summarizes the behavior [Mn(Nb6O19)2]12− and [(Nb6O19)2(Pt(OH)2)2]12− in basic solutions at pH 12. After heating of [Mn(Nb6O19)2]12− for 20 min at 60° in the presence of NaOH new very unstable phase Na8[Nb6O19]·25H2O (1) was obtained. Gentle (60 °C) heating of a solution of [(Nb6O19)2(Pt(OH)2)2]12− for 10 min in the presence of NaOH causes rearrangement into [Pt(Nb6O19)2]12−, which was isolated as Na12[Pt(Nb6O19)2]·52H2O (2). Longer heating yields white insoluble amorphous precipitate and crystals of a new hexaniobate phase K5Na3[Nb6O19]·9H2O (3). Crystal structures of the products and their structural relationship with known analogs are discussed.