Powders of REMO4 (RE: rare earth, M: Nb and Ta) were synthesized by a coprecipitation method employing water-soluble peroxo Nb/Ta complexes. This peroxo-based method is an aqueous solution route free from organics and halides. REMO4 was directly crystallized by annealing the precursor under moderate conditions. It has been revealed that the precursors are also useful for synthesis of oxynitrides under mild nitriding conditions.