The superconducting properties of heat-treated mixtures of Y1Ba2Cu3O9−x (A) and Y1.8Ba0.2Cu1O5−y (B) have been studied. As the concentration of B increases, the Meissner effect decreases, while the resistance jump near the superconducting transition increases. When the fraction of B approaches about 0.5, the superconducting transition disappears. Four different phases are identified in the mixture from the x-ray spectra, i.e., Y1Ba2Cu3O9−x, Y2Ba1Cu1O5–z, Y2Cu2O5, and Y2O3. The results agree well with the published phase diagram of this system.