Abstract The superconducting properties of the Y(Ba 1− y La y ) 2Cu 3O 9−δ phase (note La solely substituting Ba) are studied over the entire homogeneity range, viz., 0.00 < y < 0.36(2) and 1.69(2) < δ < 3.00(2) for well-defined samples prepared by the citrate gel method and annealed at controlled partial pressures of O 2. Superconductivity (for T c > 5 K, by ac susceptibility) is observed over a large range of compositions. In terms of oxygen content, the lower limit for superconductivity varies from 9 − δ ≈ 6.3 for y = 0.00 to 9 − δ ≈ 7.05 for y = 0.20. T c increases with increasing oxygen content (referring to the oxygen saturation limit 9 − δ = 6.95(1) + y for 320°C at P O 2 = 100 kPa), irrespective of the linkage of the copper-oxygen square chains when 9 − δ exceeds 7. For y = const., T c is hence correlated with the overall hole concentration p = (5 − 2δ − 2y) 3 per (CuO) p− , and these correlations are in turn related to deformation of the structure. For p = const., T c decreases with increasing y, and a hole transfer from the Cu(2) to Cu(1) coordination sphere appears to be composed of: (i) a large contribution due to an increased number of oxygens bonded to Cu(1), (ii) a smaller contribution associated with a decrease in orthorhombic deformation, and (iii) an even smaller contribution [about half and of opposite sign to (ii)] attached to decreased tetragonal deformation. The direction of the hole transfer upon a change in the tetragonal deformation depends on the ratio of the partial deformations of the Cu(1) and Cu(2) coordination polyhedra in the crystallographic c direction, the condition of no hole transfer being established when the Cu(2) pyramid deforms about 7 times faster than the Cu(1) square. On the other hand, an increase in the orthorhombic deformation always causes a Cu(1) to Cu(2) charge transfer. All these effects are overshadowed when p changes with the oxygen content. The effects of La substitution are compared with those of Sr substitution, application of external pressure, and change in oxygen content. The occurrence of superconductivity is correlated with fractional occupancy of a single, non-bonding orbital leading to a metallic state, and this picture is discussed for various oxide systems.