CASSCF, MRMP2, MR-CISD, CCSD(T) and CR-CCSD(T) theories were adopted to study the isomers of Al2O2 and its anion. CR-CCSD(T) and MR-CISD consistently indicate that the linear isomer 2 is the ground state, which is consistent with matrix isolation experiments. All three peaks at 1.88, 2.37 and 5.1 eV in the photoelectron spectra can be readily assigned as B-2(3u) -> (1)A(g), B-2(3u) -> B-3(3u) and (2)b(3u) -> B-1(3u) of isomer 1, respectively indicating that the electronic states of isomer 1 are in better agreements with the photoelectron detachment spectra. These contradicting results may imply that the major isomer of AbO, can depend on the experimental conditions. (c) 2005 Elsevier B.V. All rights reserved.