The 351 nm photoelectron spectrum of BO- is reported. Detachment to form (2) Sigma(+) BO is observed, and the electron affinity of BO is determined to be 2.508 +/- 0.008 eV. From the photoelectron spectra, vibrational frequencies of 1875 +/- 30 cm(-1) and 1665 +/- 30 cm(-1) are obtained for <(X)over tilde BO> and BO-, respectively, and the bond length in BO- is found to be 1.236 +/- 0.010 Angstrom. The measured EA is used to derive a bond energy in BO- of 9.39 eV. High-level molecular orbital calculations of the electron affinity are reported. The results from multiconfigurational SCF, G2, and complete basis set calculations are in good agreement with experiment.