The internal rotation of oxalyl chloride is investigated through ab initio Hartree-Fock and density functional theory methods. The most stable conformation is planar trans and a quasi-stable gauche isomer appearing about midway between the trans and cis isomers was also detected by the two methods. Stabilization of trans isomer is not only due to strong overlaps but also to through space interactions whereas the gauche conformation appears to be mainly stabilized by through bond interactions. In addition, the energy barrier at the cis conformation has been found to be mainly due to through space interactions. (C) 2003 Elsevier B.V. All rights reserved.