The properties of the YBa2Cu3O7−x(‘‘123’’) superconducting systems are generally determined by metal ratios, crystal structure and oxygen concentration. In order to obtain the correct oxygen content and crystal structure, it is normally necessary to subject the material to a high‐temperature annealing process in which the temperature and oxygen pressure must be controlled in order to avoid oxygen loss and prevent the formation of second phase material, while maintaining the correct cation stoichiometry. For materials with the optimal cation stoichiometry, poor performance as a superconductor is generally assumed to be the result of improper annealing. Recently, we have performed a detailed study of the resistivity of an optimal ‘‘123’’ ceramic system, as measured by a four‐point probe in the presence of controlled atmospheres of oxygen and other gases ranging in pressure (at room temperature) from ∼1 mTorr to 70 Torr. These studies were conducted in a closed cycle He refrigerator, during both cooling and heating cycles.Cooling the ‘‘123’’ material, in the presence of an ambient oxygen background equivalent to several Torr at room temperature, produced a metallicRversusTbehavior withT0(onset) of ∼103 K andTcof ∼91 K. As long as this atmosphere was maintained above the ‘‘123’’ material, only minor differences were detected in the resistance versus temperature characteristic upon reheating and repeated temperature cycling. Lowering the oxygen pressure, followed by repeated temperature cycling however, produced a continuous reduction inTcto an eventual value of ∼60 K at 1 mTorr. Reintroduction of various doses of air or O2(at any point in the cycle) immediately increasedTc, with apparent total restoration to the optimal resistance values at ∼5 to 12 Torr. Atmospherically controlled changes of oxygen concentration or reordering of oxygen vacancy sites are suspected to be responsible for these variations inTc, which occur with out annealing and with no apparent changes in structure.A variety of other reproducible alterations in theRversusTbehavior of this material (including unique humps and shifts) have been induced (and removed) through controlled changes in the ambient atmosphere. There are reasons to suspect that these surface‐oriented effects originate in the grain boundaries of these materials.