We report an investigation of possible oxygen loss from an ion-bombarded YBa2Cu3O7-x superconductor. The chain oxygen sites in thin-film samples were filled with 18O, and the 18O content of the films measured as a function of irradiating ion dose using the 18O(p,α) 15N nuclear reaction. Absolute measurement of the 18O concentration was achieved by the use of a Si 18O2 standard. No significant change in the 18O concentration was detected after irradiation with 2×1014 2 MeV As ions/cm2, which is a dose sufficient to raise the resistivity of the samples by a factor of 10 4. We conclude that ion irradiation does not cause oxygen to outgas from the chain sites. This implies that ion bombardment effects in YBa 2Cu3O7-x are due to radiation-induced defects.