Alkali tantalates ATaO(3) and alkali niobates ANbO(3) (A = Na and K) powders with perovskite structure were synthesized by the hydrothermal method. The band gaps estimated of niobates are smaller than those of tantalates. This factor contributes to red shift in absorbance from tantalates to niobates in diffuse reflection spectra. Their photocatalytic activities were investigated. The order of the activities is NaTaO3 >> KTaO3 > NaNbO3 > KNbO3. The cubic crystalline structure of NaTaO3 powder shows the highest photocatalytic activity. The effect of the amount of NaOH in the NaTaO3 preparation upon the photocatalytic activity was studied. Scanning electron microscopy indicates that the appropriate excess alkali in the NaTaO3 preparation can improve the crystallinity of NaTaO3 and suppress formation of defects in NaTaO3 powder, resulting in an increase in the photocatalytic activity. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.