Substitution of Ca(II) for Y(III) into tetragonal YBa2Cu3O6 has been achieved and a superconducting transition has been observed for a material of nominal composition (Y0.5Ca0.5) Ba2Cu3O6 (Tc (onset) ˜50K) . Observations using high resolution electron microscopy show samples with x < 0.3 consist of a complex mixture, including (Y, Ca) Ba2Cu3O6+δ, YBa2Cu3O6+δ, and BaCuO2. Further, structural refinement using neutron diffraction data provide evidence of a solid solution limit at x ˜ 0.3. A direct analogy can be drawn between superconducting (Y1−xCax)Ba2Cu3O6 (0.1 < x < 0.3) and superconductors of the type (Y1−xCax)Pb2Sr2Cu3O8 and (La2−xM(II)x)CuO4. Substitution of the Ca(II) for Y(III) effectively increases the formal copper oxidation state. The refined structural model is fully consistent with partially oxidized CuO2 sheets separated by linear O-Cu(I)-O units.