Alkali niobate, K1-xNaxNbO3, nanoparticles (ca. 50-60 nm) were continuously synthesized from Nb-sol and NaOH/KOH solutions with a supercritical water flow system. The syntheses were carried out at temperatures from 400 to 480 degrees C, at pressures from 25 to 30 MPa, and at reaction times from 0.1 to 8.3s. X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy-energy dispersion spectroscopy (SEM-EDS) techniques were used to characterize the samples. Single phase KNbO3 was obtained at 480 degrees C and 25 MPa at an initial K/Nb molar ratio of 15. The K/Nb molar ratio of the products increased with an increase in the reaction time. For NaNbO3, single phase product was formed under supercritical water conditions irrespective of the temperature and pressure. The K1-xNaxNbO3 nanoparticles were synthesized at 480 degrees C and 25 MPa for varying Na/K concentrations. Single phase K0.5Na0.5NbO3 was obtained at an initial Na/K molar ratio of 0.25. To form the perovskite structure, the rearrangement of NbO6 units with alkaline ions is required. The rearrangement proceeds more rapidly with Na+ ions than with K+ ions probably due to the smaller ionic radius of Na+. Therefore, formation of K0.5Na0.5NbO3 was achieved at the excess amount of K+ ions than Na+ ions. (C) 2015 Elsevier B.V. All rights reserved.