The superconducting transition temperature and the orthorhombic distortion in YBa2Cu3O7-δ are found to be determined mainly by the oxygen occupancy of the Cu-O chains and may explain the discrepancies between experiments with equal total oxygen stoichiometry. These results are obtained from simultaneous gas evolution, superconductivity and X-ray diffraction experiments geared towards understanding the role of the oxygen ordering in ceramic superconductors. The evolution kinetics is found to proceed by an initial large change followed by a constant chain occupancy. The superconducting transition temperature follows the change in chain occupancy and clear evidence is presented for the occasional presence of two distinct superconducting oxygen phases which are difficult to uniquely establish from scattering experiments. The presence of these phases is critically dependent on details of the preparation condition. © 1989 IOP Publishing Ltd.