Peaks due to M– H₂O are observed in the mass spectra of aromatic carbonyl compounds which contain an ortho(or peri) methoxy-substituent. This process is of diagnostic value for simple systems, but cannot be extended to those systems which have a large number of oxygen-containing substituents. The fragmentation has been studied by ²H and ¹⁸O labelling. The properties of the M– 18 fragment in the spectrum of o-methoxybenzaldehyde correspond to those of a benzofuran radical ion.