Abstract We report bulk superconductivity with a maximum critical temperature T c of 20 K in single-phase Y 1− z Ca z Sr 2Cu 2.5Fe 0.5O 6+ x (0.05≤ z≤0.18) synthesized under near ambient oxygen pressure. Materials were characterized by X-ray powder diffraction, iodometric titration, magnetic susceptibility, room-temperature 57Fe Mössbauer absorption spectroscopy, and transport measurements. With increasing z, the oxidation state of the Fe-Cu-O array remains essentially constant. Five-fold coordinated Fe in the Cu(1)O x planes appear to be intermediate-spin Fe 3+, all others are present as high-spin Fe 3+. The concentration of Fe 3+ ions in the Cu(2)O 2 sheets increases gradually with increasing z. We interpret the development of a broad maximum versus z near z=0.12 in T c and the Meissner fraction V f to be due to an increasing transfer of holes from the reservoir in the Cu(1)O x planes to the Cu(2)O 2 sheets as the Coulomb repulsion of the positively charged Y 1− z Ca z plane is reduced.