Phase transformations in Pb-2212 and Pb-2223 compositions contained in the isopleth defined by the formula (Bi, Pb) 2 Sr 2 Ca n - 1 Cu n O 2 n + 4 + δ were investigated by thermal analysis of the oxygen activity and of the electrical resistance, inside an electrolytic cell of CaO-stabilized zirconia.These methods of thermal analysis allow us to not only determine the main known phase transformations in these compositions but also new transformations in the system close to 750°C and 500°C under p(O 2 ) = 10 5 Pa pure oxygen atmosphere. Our e.m.f. thermal analysis shows the existence of a strong redox equilibrium spread between 300 and 550°C which was also detected by the resistance analysis and cannot be interpreted by a phase transformation. X-ray diffraction analysis has proved that no structural transformation appears below 550°C. The results obtained in both parts of the 750°C transformation by X-ray diffraction of quenched powders are still too incomplete to conclude that the Majewski eutectoid transition: 2212 2201 +12 Ca 2 CuO 3 + 12 CuO occurs at this temperature but this transformation might correspond to (Ca, Sr) 2 PbO 4 precipitation.Both the new redox transformations observed arround 750 and 500°C might also be valence transitions, inside the high-T c superconducting phases.