The unit cell of Pb 2Sr 2YCu 3O 8+∂ with ∂ = 0 contains the sequence of layers: …(Y)(CuO 2)(SrO)(PbO)(Cu)(PbO)(SrO)(CuO 2)… in which the (Y) and (Cu) planes are oxygen deficient. Because of the low formal valences of the Cu cations, (Cu 2+in the (CuO 2) planes and Cu 1+ in the (Cu) planes) this compound is not a superconductor. Neutron, x-ray, and electron diffraction measurements show that the Cu layer inserted between two (PbO) layers can easily incorporate extra oxygen. A theoretical composition ∂ = 2 is possible, although in practice only a stoichiometry corresponding to ∂ = 1.9 has been achieved so far. This may be explained by the presence of twin boundaries. In the range of composition 0≤∂≤1, mixtures of two phases are obtained, one with ∂ = 0 and the other with ∂ = 1, whose relative quantities depend on the total amount of oxygen incorporated by the sample. The positive charges induced in Pb 2Sr 2YCu 3O 8+∂ by oxidation, are localized on the Pb sublattice. This localization hinders the charge transfer to the conducting (CuO 2) planes and, for this reason, no superconductivity is present in oxidized samples. Pb 2Sr 2YCu 3O 8+∂ becomes superconducting at ≈ 80K when some of the trivalent Y cations are replaced by divalent Ca. In this case the extra positive charges oxidize the Cu 2+ cations of the (CuO 2) planes to 3+ instead of the Pb 2+ to 4+ and the Cu 1+ to 2+, as does the incorporation of oxygen. When heat-treated at 450 °C in O 2, Pb 2Sr 2Y 1−xCa xCu 3O 8+∂ behaves similarly to the undoped compound. The oxygen uptake suppresses the superconducting transition which is re-established by heattreating the sample at same temperature in N 2.