The crystal growth of (VO)(2)P2O7 single crystals is discussed with respect to the formation of a high-temperature nonstoichiometric phase (VO)(2)P2O7+x. The interaction between vanadyl pyrophosphate and oxygen was investigated in the temperature region 600-900 degreesC. It was found that the decay of the high-temperature nonstoichiometric phase at lower temperature led to imperfections in crystals grown in an atmosphere with oxygen concentration higher than 0.2%. Two mechanisms for the decay are considered. The oxygen content of the crystals may be reduced at the surface by evolution of gaseous oxygen, whereas in the bulk the formation of an oxygen-enriched solid phase is observed. (C) 2000 Elsevier Science Ltd. All rights reserved.