In the pyrolysis of pure acetaldehyde at 450–525 °C it was found that ketene, although stable under these conditions, is not formed as a final reaction product. This result requires a modification of the mechanism of Liu and Laidler, according to whom the ethenoxy radical CH2CHO splits into CH2CO and H. It is concluded that instead of splitting unimolecularly, CH2CHO disappears mainly through the reaction[Formula: see text]and that it also undergoes the process[Formula: see text]