The variations of positron lifetimes with time at room temperature were measured for the Bi- based 2212 phase superconductor. lt is found that lifetimes and

intensity I1

mean lifetime and bulk lifetime all keep on decreasing for 130 days

and then remain roughly unchanged. This result confirms that the structure of oxide superconductors is instable even at room temperature. The decrease of the annihilation parameters with time is explained in terms of the intercalation of the excess oxygen in the region between the Bi-O layers. Possible correlations with the electron structure

oxygen content

hole density and transition temperature are also discussed.