We have investigated the chemical bonding states at the surfaces of La_1.5Ba_0.5SrCu₂O_6-d annealed under CO, CO₂ and SO₂ gaseous conditions, by using X-ray photoelectron spectroscopy (XPS). The C 1s and S 2p core-level data obtained from XPS indicate that each adsorbate exists as CO₃^δ- and SO₄^δ- on the reaction surfaces, respectively. The Cu 2p XPS spectra suggest that the reaction surfaces of the cuprate donate the electrons to the adsorbates and that the hybridizations of Cu(3d)-O(2p) orbitals change.