The collisionally activated dissociation of a variety of isomeric disubstituted aromatic ions formed by ion-molecule reactions were examined in order to characterize ortho effects in closed-shell systems. Closed-shell ions of metboxyacetophenooe, hydroxyacetophenone, methoxyphenol, anisaldehyde and hydroxybenzaldehyde were formed by proton transfer, methyl addition or methyne addition by using dimethyl ether or ethylene oxide as chemical ionization reagents, and then the structures of these adducts were studied by deuterium-labelling methods and by collisionally activated dissociation techniques in a triple quadrupole mass spectrometer or a quadrupole ion trap. Typically, the metu and para isomers have qualitatively similar dissociation spectra which reflect the types of dissociation reactions observed for the corresponding monosubstituted aromatic ions. The predominant dissociation pathways of the [ M + H] + and [ M + 151 + ions are directed by the electron-withdrawing substituents, whereas the major dissociation pathways of the [M + 131' ions are related to the electron-releasing substituent. In contrast, the dissociation routes of the corresponding ortho isomers are dramatically different. This is attributed to the opportunity for functional group interactions of the ortho isomers which facilitate alternative pathways.