Hydrothermal synthesis is a well-established method to produce complex oxides, and is a potential interesting approach to synthesize stoichiometric lead-free piezoelectric K0.5Na0.5NbO3. Due to challenges in obtaining the desired stoichiometry of this material, more knowledge is needed on how the end-members, KNbO3 and NaNbO3, are nucleating and growing. Here, we report on the formation mechanisms and growth during hydrothermal synthesis of KNbO3 and NaNbO3 by insitu synchrotron powder X-ray diffraction. We show that tetragonal KNbO3 crystallites form from dissolved T-Nb2O5 at 250 degrees C-300 degrees C and 250bar while orthorhombic NaNbO3 forms via several crystalline intermediate phases at 225 degrees C-325 degrees C and 250bar. The crystallite size of KNbO3 is decreasing while the crystallite size of NaNbO3 is increasing with increasing temperature, demonstrating that the presence of intermediate phases is highly important for the nucleation and growth of the final product. The different crystallization schemes explain the challenge in obtaining stoichiometric K0.5Na0.5NbO3 by hydrothermal synthesis.