The singlet potential energy surface of the M(2)O(2) (M = Al, Ga, In, Tl) system has been investigated using ab-initio electronic structure theory at the SCF, MP2, CISD, QCISD, and CCSD(T) levels and the density functional theory (for Al2O2 and Ga2O2). The results show that for Al2O2 and Ga2O2 accurate prediction of the relative stability of the various isomers can only be obtained at the correlated level using large one-particle basis sets. At the CCSD(T)/TZ2P+f level, the most stable isomer of Al2O2 is a cyclic D-2h form, whereas CISD and QCISD results indicate that the linear MOMO (C-infinity nu) structure is the global minimum for Ga2O2. The results of ab-initio calculations with relativistic effective core potential also establish the linear MOMO geometry to be more stable (relative to the cyclic structure) for M = In, Tl. The energy difference between the cyclic D-2h form and the Linear MOMO (C-infinity nu) is less than 12 kcal/mol for all the systems studied. Theoretically predicted harmonic vibrational frequencies agree very well with the latest available experimental data.