Cyanobacterial aldehyde decarbonylases (ADs) catalyze the conversion of Cn fatty aldehydes to formate (HCO2–) and the corresponding Cn-1 alk(a/e)nes. Previous studies of the Nostoc punctiforme (Np) AD produced in Escherichia coli (Ec) showed that this apparently hydrolytic reaction is actually a cryptically redox oxygenation process, in which one O-atom is incorporated from O2 into formate and a protein-based reducing system (NADPH, ferredoxin, and ferredoxin reductase; N/F/FR) provides all four electrons needed for the complete reduction of O2. Two subsequent publications by Marsh and co-workers [Das, et al. (2011) Angew. Chem. Int. Ed.50, 7148−7152; Eser, et al. (2011) Biochemistry50, 10743–10750] reported that their Ec-expressed Np and Prochlorococcus marinus (Pm) AD preparations transform aldehydes to the same products more rapidly by an O2-independent, truly hydrolytic process, which they suggested proceeded by transient substrate reduction with obligatory participation by the reducing system (they used a chemical system, NADH and phenazine methosulfate; N/PMS). To resolve this discrepancy, we re-examined our preparations of both AD orthologues by a combination of (i) activity assays in the presence and absence of O2 and (ii) 18O2 and H218O isotope-tracer experiments with direct mass-spectrometric detection of the HCO2– product. For multiple combinations of the AD orthologue (Np and Pm), reducing system (protein-based and chemical), and substrate (n-heptanal and n-octadecanal), our preparations strictly require O2 for activity and do not support detectable hydrolytic formate production, despite having catalytic activities similar to or greater than those reported by Marsh and co-workers. Our results, especially of the 18O-tracer experiments, suggest that the activity observed by Marsh and co-workers could have arisen from contaminating O2 in their assays. The definitive reaffirmation of the oxygenative nature of the reaction implies that the enzyme, initially designated as aldehyde decarbonylase when the C1-derived coproduct was thought to be carbon monoxide rather than formate, should be redesignated as aldehyde-deformylating oxygenase (ADO).