The influence of preparation conditions and microstructure on the superconductive properties of single-phase poly-crystalline YBa2Cu3O7−x was investigated by electron probe microanalysis, transmission electron microscopy (TEM), and x-ray powder diffraction as a function of temperature in various ambients supplemented by resistivity and susceptibility measurements. Leaching of Ba was observed when samples were brought in contact with water. The TEM imaging revealed that individual grains have an extremely defect-rich outer shell and an inner core with a domain structure with a and b axes interchanged. The transition temperature Tc was found to decrease with increasing quench temperature in the range 400–900°C. The Tc was observed to be linearly proportional to the difference in the orthorhombic cell parameters (b-a). Further implications are discussed.