The rotational isomerism of alpha-fluoroacetaldehyde (FAL), alpha-chloroacetaldehyde (CAL) and alpha-bromoacetaldehyde (BAL) has been studied through ab initio and density functional theory. Potential energy surfaces have been obtained and the equilibrium was found to be between the cis and trans forms for FAL and between the cis and gauche forms for CAL and BAL. The trans or gauche conformer was found to be more stable in the three cases. Natural bond orbital analysis was conducted and led to the conclusion that steric repulsion prevail over electron delocalization in determining the conformational preference. (C) 2004 Elsevier B.V. All rights reserved.