The reactions of aldehydes on metal oxide surfaces have exposed some of the most exquisite chemistry in comparison to their alcohol and carboxylic acid counterparts. Due to the intermediate oxidation state of the carbonyl carbon on an aldehyde, reactions such as the Cannizzaro disproportionation (forming alkoxides and carboxylates) and Tishchenko (forming ester) have been observed on various metal oxide surfaces, in addition to direct oxidation and reduction. Furthermore, condensation reactions have also been observed with aldehydes possessing alpha-H atom(s) over stoichiometric metal oxide surfaces, while surface defect sites are able to reductively couple two aldehyde adsorbates to yield alkene. Herein, we present the reactions of acetaldehyde over the stoichiometric, electron-irradiated, H-2-reduced, and argon ion-sputtered surfaces of a UO2 (1 1 1) single crystal. Temperature programmed desorption results over the four surfaces show evidence of benzene formation, and it appears that electron-irradiation slightly increases the conversion to benzene, H-2 reduction doubles it, and argon ion-sputtering increases it by fourfold. Thus the amount of benzene formed from acetaldehyde over a UO2 single crystal might be a novel way to quantitatively estimate the extent of surface defects.