The geometries, vibrational spectra, and relative energetics of CH3OBr and CH3BrO have been examined using various ab initio and density-functional methods. The estimated heat of formation of CH3OBr, at 0 K, is -5.4 kcal mol(-1), in good agreement with the estimates of bond additivity. The CH3BrO isomer is 42.6 kcal mol(-1) higher in energy than CH3OBr.