The resistivity of Ba2YCu3Oxwas measured in the 450–850 °C range at various oxygen partial pressures. The tetragonal to orthorhombic phase transition is observed as a change in the slope of the resistivity curve. The lack of a discontinuity in the curve is consistent with the phase transition being higher than first order. It was found that decreasing the oxygen contentxbelow 6.2 changed the resistivity of the tetragonal phase from metallic character to semiconducting character. In the metallic region of the tetragonal phase the superconducting transition temperature is depressed below 60 K, while in the semiconducting region, no superconducting onset was observed down to 6 K. In the orthorhombic phase, the resistivity of the material is shown to be governed by the number of oxygens per unit cell, suggesting that the resistivity can serve as a quality control parameter in the production of superconducting devices.