The crystal growth of two uranium-containing oxides, K2UO4 and Na4UO5, utilizing hydroxide melts is reported. Our group has been investigating the crystal growth of complex oxides with new compositions and structure types using molten hydroxide fluxes. In this paper we describe the application of this method to uranium oxides, where the dissolution of the uranium cations in hydroxide melts and subsequent re-crystallization leads to the formation of alkali metal-uranium oxides. This is the first report of the use of molten hydroxides as a medium for crystal growth of uranium-containing oxides. (C) 2009 Elsevier B.V. All rights reserved.