Using high level ab initio molecular orbital methods, theoretical studies were performed on the low-lying excited states of CH 3 OBr. The complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) methods, with the cc-pVTZ basis set, were used to calculate the vertical excitation energies for the lowest three 1 A ' , two 1 A'', two 3 A ' , and two 3 A'' states. The CASSCF method was used to obtain the potential energy curves for the lowest three 1 A ' , two 1 A'', two 3 A ' , and two 3 A'' states of CH 3 OBr along the O-Br coordinate. All of the eight excited states were found to be strongly repulsive along the O-Br coordinate.