The possible reaction paths corresponding to the catalyzed and uncatalyzed reaction between chloroketene (as a model activated ketene) and acetaldehyde (as a model alkyl carbonyl compound) have been studied using ae, initio methodologies and taking into account solvent effects. Eight transition structures have been located and characterized, both in the gas phase and in solution. It has been found that formation of trans-2-oxetanones is favored in the absence of catalyst, whereas the catalyzed reaction leads to the preferential formation of the corresponding cis cycloadducts. These results are in qualitative agreement with the experimental data available.