Synthesis of potassium titanoniobate (KTiNbO5) powder was achieved by the hydrothermal process in supercritical and subcritical water using Nb2O5 fine powder, titanium tetra-isopropoxide and potassium hydroxide aqueous solution as starting reactants. The novel hydrothermal process for synthesizing KTiNbO5 powders employs significantly reduced reaction temperature (300-400 degrees C) in comparison with the conventional solid-state synthesis and polymerizable complex methods (1100 and 700 degrees C), to get an advantage in the preparation of multicomponent oxide compounds such as KTiNbO5 with pure phase, thin rectangular particles and large surface area. Hydrothermal temperature, time duration and alkaline concentration employed in the synthesis determined the structure and particle morphology of the final products. The hydrothermally synthesized KTiNbO5 powders with ruthenium loading were used for the photocatalytic hydrogen evolution from aqueous methanol solution. The surface area is responsible for the highly photocatalytic performance of ruthenium loaded KTiNbO5 powders hydrothermally synthesized in supercritical and subcritical water. (C) 2006 Elsevier B.V. All rights reserved.