Investigations of the CaO-V2O3-VO2-V2O5 system under various oxygen pressure and with addition of sodium in starting mixture are described. A diagram of CaV4O9 and CaV3O7 stability and the reaction CaV3O7+VO2<---->CaV4O9 have been studied depending on temperature and oxygen partial pressure. As a result, single-phase CaV4O9 and CaV3O7 ceramics were prepared, and plate-like single crystals of CaV4O9 were grown. Magnetic susceptibility data confirmed the existence of a disordered spin liquid state with spin gap Delta = 100 +/- 15 K in CaV4O9 and of an antiferromagnetic order below T-N = 22K in CaV3O7. (C) 2002 Elsevier Science B.V. All rights reserved.