The dissociative ionization of sixteen 4-benzoyl- and 4-benzylpyridine derivatives and their deuteroanalogs has been studied. An ortho effect, due to the benzoyl and benzyl radicals in the methyl group in the 5-position of the pyridine ring, has been detected. It has also been established that fragmentation of 4-benzoylpyridines substituted with a nitro group in the benzene ring leads to [M-OH]+ ions, due to the ortho effect, whereas fragmentation of 4-benzylpyridines leads to [M-C6H5R]+ ions. The probability of a given process depends on the position and nature of any substituent in the benzene ring; this makes it possible to identify different isomers in a given series of compounds. © 1987 Plenum Publishing Corporation.