Nanocrystalline potassium niobates (KNbO3, KNb3O8 and K4Nb6O17) of different morphologies were synthesized with the low-temperature hydrothermal method by adjusting the alkalinity of the reaction system. A possible formation mechanism of the nanocrystalline niobates was proposed based on the experimental results. All of the products were characterized with X-ray powder diffraction and transition electron microscopy. The Fourier transform (FT) infrared, FT-Raman and UV-VIS properties of the niobates were studied. (C) 2002 Elsevier Science B.V. All rights reserved.