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High-quality Bi-2212 single crystals with a transition temperature of 91 K were annealed in vacuum at 550 oC for 10, 20, 40 and 72 h. AC susceptibility measurements showed that Tc and Tc increased with increasing annealing time ta in the case of short-time annealing (ta40 h). However, as the annealing time was further increased to 72 h, the samples became non-superconducting. The results of X-ray diffraction showed that the lattice parameter of the crystal in c-axis increased with the annealing time, indicating the decrease of the oxygen content. Raman spectra revealed that the intensities of the peaks at 290 and 650 cm - 1 corresponding to the OBi vibration modes were gradually enhanced with the increase of annealing time as well as with the decrease of oxygen content, which supported the model that the excess oxygen atoms were located in the bismuth layer interstitially.