The rotational spectrum of the OBrO radical has been observed in the gas phase over the solid products of the O+Br-2 reaction. Spectra have been measured for both (OBrO)-Br-79 and (OBrO)-Br-81 in their (000), (010), and (020) vibrational states in selected regions between 88 and 627 GHz spanning the quantum numbers 1 less than or equal to N less than or equal to 61 and 0 less than or equal to K-a less than or equal to 14. The spectra are well described by a Hamiltonian which includes centrifugal distortion effects for fine and hyperfine terms. The molecular structure, the dipole moment, and the harmonic force field have been derived, and they, as well as fine and hyperfine structure constants, are compared with data of related molecules and electron spin resonance data from OBrO isolated in cryogenic salt matrices. (C) 1997 American Institute of Physics. [S0021-9606(97)01544-4].