The effect of annealing conditions on the superconductivity of the Pb-based 1 2 1 2 phase Pb0.65Sr2.35Y0.5Ca0.5Cu2Oy system was studied. The weight change of the samples was measured by thermogravimetric analysis !TGA). The phases occurring in the samples were detected by X-ray diffraction (XRD) and their superconducting transitions were determined by alternating current (a.c.) susceptibility measurements. The results showed that the oxygen occupancy sites of the samples annealed under various conditions were different, and the oxygen content or oxygen occupancy site affected the phase formation of the sample. It has been found that there is an optimum value of oxygen content at which T-c is the highest, and the highest T-c can be obtained only within a very narrow range of oxygen content.