The quatenary phase relations near YBa2Cu3O6+x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu2O2. Nine invariant reactions were discovered, including decomposition of YBa2Cu3O6+x into Y2BaCuO5, BaCu2O2 and YBa3Cu2O6+y. The lower limit of oxygen partial pressure at which pure YBa2Cu3O 6+x is stable is 4.0×10−4 atm O2 at 850°C; the lower limit increases in the presence of BaCuO2 and Cu2O impurities. These results indicate that the temperatures used for single crystal growth of YBa2Cu3O6+x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.