AbstractThe behaviour of 3′,4′‐dihydroxybenzyl fatty acid ester towards the NH3/NH 2−system has been investigated under negative‐ion chemical ionization (NICI) conditions. Under NICI, proton abstraction takes place regioselectively at one phenol site rather than from the enolizable position. Analysis of specific fragmentations of the isotopically labelled phenoxide species prepared in the gas phase by ND3indicates that isomerization into an ion‐dipole intermediate (via charge‐promoted cleavage) takes place prior to fragmentation. Its dissociation provides fatty acid carboxylate ions (the charge is stabilized by coiling of the side chain). The acidity of the phenol group must be enhanced by the presence of a second group at the positionorthoto the first. This explains why the previous ion‐dipole complex isomerizes by proton transfer into a second isomeric form which decomposes yielding the phenoxide species, as shown by labelling experiments. Stabilization of the negative charge by hydrogen bonding (ArO−…︁H+…︁O−Ar) from the phenoxide form is possible. A similar situation characterizes 3′,4′‐dihydroxybenzyl phenyl ethyl ester and 3′,4′‐dihydroxybenzyl benzyl ester. Their behaviour was also studied to find the influence of the side chain structure on the pathway of ion‐dipole dis