Crystallization of Bi2Sr2Ca1Cu2O8+&dgr;has been studied as a function of the partial pressure of oxygen, usinginsituhigh temperature x‐ray measurements, in order to determine the phases that form and the kinetics of growth. As the partial pressure of oxygen is systematically reduced, the dominant second phase changes from (Sr,Ca)2Cu1O3at 150 Torr of oxygen, to (Sr,Ca)1Cu1O2at 15 Torr of oxygen, to CaO in a pure nitrogen atmosphere. Normally, in 150 Torr of oxygen, Bi2Sr2Cu1O6+&dgr;is the first Bi cuprate to form from the melt but if the oxygen partial pressure is reduced to 75 Torr, then Bi2Sr2Ca1Cu2O8+&dgr;is favored due to the decreased amount of 2:1 phase in the melt.