NaxK1-xNbO3-based solid solutions are one of the leading lead-free piezoelectric materials that exhibit comparable piezoelectric properties to actuator grade lead-zirconium-titanate (PZT). Here we report the growth of single phase, epitaxial NaxK1-xNbO3 (NKN) films hydrothermally grown over single crystal Nb: SrTiO3 substrates in a simple KOH and NaOH mixture with Nb2O5 as a precursor powder. The epitaxial NKN film remains single phase up to 13.3% NaOH relative concentration (compared to the total hydroxyl ion), above which a second layer of Na-rich NKN phase grows on top of the original NKN layer. It is discovered that by controlling the reaction time and applying mild post-growth treatment, the unwanted Na-rich phase growth can be suppressed, therefore enabling epitaxial growth of single phase NKN at compositions closer to the morphotropic phase boundary. (C) 2011 Elsevier B.V. All rights reserved.