We investigate a typical trihalide ionic liquid, 1-octyl-3-methylimidazolium tribromide, using X-ray photoelectron spectroscopy. The electronic environment of the anion is analyzed in detail. The Br 3d binding energies of the two components are both higher than that measured for 1-octyl-3-methylimidazolium bromide, due to the delocalization of the negative charge spreading over three atoms within the anion. The basicity of [Br3]- is estimated based upon N 1s binding energy. It is found similar to that of [BF4]-.