Implantation studies with16O and18O isotopes have been used to evaluate a recent proposal that an 889 cm−1absorption band in O‐containing Si is produced by two oxygen atoms in a vacancy. The present isotope studies indicate that the 889 cm−1mode is determined by a single oxygen atom bonded to Si. Thus, O‐O interactions in the 889 cm−1mode must be weak or nonexistent. Possible relationships of the 889 cm−1band to oxygen‐vacancy defects and thermal donors are considered.