The effect of oxygen partial pressure on the stability of the highTcphase (>100 K) in a Bi‐Pb‐Sr‐Ca‐Cu‐O system has been studied using resistivity, x‐ray diffraction, and ac susceptibility measurements. It was observed that part of the highTcphase formed by sintering at 840 °C in a low oxygen partial pressure of 1/10 atmosphere was transformed into a lowTcphase (∼80 K) and an insulating phase by heating at the same sintering temperature in O2of one atmosphere. The original highTcphase was restored upon retreatment at the same temperature in a low oxygen partial pressure of 1/10 atmosphere.