Phase-pure tantalum oxynitride (beta-TaON) powders were synthesized by thermal ammonolysis of Ta2O5 powders. X-ray diffraction revealed an enlargement of the unit cell and an increase of the crystallite size with increasing ammonolysis temperature. Scanning electron microscopy showed reduced particle sizes for beta-TaON synthesized at 800 and 900 degrees C compared to the precursor oxide. With increasing nitridation temperature the Brunauer-Emmett-Teller surface area was reduced and the nitrogen content increased. UV-Vis spectroscopy showed a bandgap energy of 2.6-2.4 eV. The highest oxygen evolution rate of 220 mu mol.g(-1).h(-1) was achieved for beta-TaON synthesized at 800 degrees C. The factors determining the photocatalytic activity of beta-TaON powders were found to be the specific surface area and defects in the beta-TaON.