The effect of temperature and atmosphere on stability of the high‐Tcphase in Bi1.8Pb0.4Sr2Ca2Cu3Oxsuperconductors has been investigated using ac susceptibility, x‐ray diffraction, and Raman measurements. It is found that the high‐Tcphase becomes unstable due to the structural phase transformation at temperature above 650 °C in O2of 1 atm. The formation and the decomposition of the high‐Tcphase is found to occur through the medium of the Ca2PbOxphase and depends on oxygen partial pressure at temperature above 650 °C.