Gas chromatography/mass spectrometry was used to analyze the oxygen evolved from normal and 18O enriched YBa2Cu3O6.98 upon dissolution in normal and 18O enriched acid. From the 18O 16O ratio we show that the oxide phase is the source of all the evolved oxygen. Our results indicate that some of the lattice oxygens are in a state other than O2, probably O or peroxide. The fact that we are unable to chemically detect peroxide suggests that the radical O- could possibly be the species released from the oxide. © 1989.