The structure and charge transfer correlated with oxygen content are studied by measuring the positron lifetime parameters of the Y0.8Ca0.2Ba2Cu3Oy system with a large range of oxygen content (y=6.84–6.32). The local electron density ne is evaluated from the positron lifetime data. The positron lifetime parameters show a clear change around y=6.50 where the compounds undergo the orthorhombic–tetragonal phase transition. The effect of ne and oxygen content on the structure, charge transfer and superconductivity are discussed. With the decrease of oxygen content y, O(4) tends to the Cu(1) site, causing carrier localization, and accordingly, the decrease of ne. This would prove that the localized carriers (electrons and holes) in the Cu–O chain region have great influence on the superconductivity by affecting the charge transfer between the reservoir layers and the conducting layers. The positron annihilation mechanism and its relation with superconductivity are also discussed.