The rotational spectrum of the OIO radical has been observed for the first time. Spectra of both the ground and first two excited bending vibrational states have been analyzed. Rotational, centrifugal distortion, fine, and hyperfine constants have been derived. These constants have been used to determine the molecular structure, harmonic force field, and electron distribution. The OIO molecular parameters are compared with those of OClO and OBrO. (C) 2003 American Institute of Physics.