The loss of CHO,' from the molecular ion of phenoxyacetic acid and the expulsion of an H' atom from ionized anisole lead to phenoxymethylene ions, which fragment predominantly by CO loss on the microsecond time-scale. Carbon-13 labelling reveals that -90% of the CO molecules expelled from the metastable ions derived from phenoxyacetic acid incorporate the carbon atom from the 1-position of the phenyl group of the parent compound, whereas the residual C O molecules contain one of the other carbon atoms of the aromatic ring. The 2-fluoroand 2-methylphenoxymethylene ions derived from the appropriate aryloxyacetic acids behave similarly, i.e. the carbon atom of the methylene group of the parent compound is not incorporated in the expelled CO molecules. In contrast, -45% of the C O molecules eliminated from the metastable phenoxymethylene ions formed from ionized anisole contain the carbon atom of the methyl group, while the remaining part contains the carbon atom from the 1position of the phenyl ring of the parent compound. This result is taken as evidence for the Occurrence of a skeletal rearrangement of the anisole molecular ion leading to an interchange between the carbon atom of the methyl group and the carbon atom at the 1-position of the ring. The elimination of C O from the metastable ions generated from either phenoxyacetic acid or anisole gives rise to a composite metastable peak. Conclusive evidence as to the formation of [ C,H,O) + isomers other than the phenoxymethylene ion is not obtained, indicating that the composite metastable peak is a result of two competing reactions both leading to CO loss. Possible mechanisms of these reactions are discussed together with the mechanism of the skeletal rearrangement of the molecular ion of anisole prior to H' loss.