We have performed ion channeling characterizations on the (001) and (100) surfaces ofYBa2Cu3O7−xsingle crystals, using 950 keVHe+ions at temperatures ranging from 20 to 780 °C. In oxygen pressures below5×10−5 Torr,we directly observed the surface changes as the sample temperature was increased, and found that the compositional and structural changes occurred from around 400 °C. At higher oxygen pressures, we heated the crystals and performed channeling analysis before and after the heating. We found that theYBa2Cu3O7−xcrystal surfaces were stable at temperatures up to 780 °C in the oxygen pressure range of 1 mTorr–1 Torr. Beyond this range, the surfaces deteriorated at high temperatures. ©1997 American Institute of Physics.