The equilibrium structure and potential energy surface of dialuminum monoxide, Al2O, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with basis sets of triple- through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The vibrational-rotational energy levels of the Al-2 O-16 and Al-2 O-18 isotopic species were calculated by a variational approach. The predicted energy levels are in remarkably good agreement with the available experimental spectroscopic data (from laser-induced fluorescence), demonstrating that the Al2O molecule is linear at equilibrium in its ground electronic state. The reported theoretical data settle controversies between the experimental studies about the equilibrium structure and assignment of vibrational fundamentals of the Al2O molecule. (C) 2004 American Institute of Physics.