A study has been made of the fragmentation upon electron impact of 3,3‐dimethyl‐2,3,4,5,10,11‐hexahydro‐11‐phenyl‐1H‐dibenzo[b,e][1,4]diazepin‐1‐one and fifteen of its derivatives containing chloro, bromo, methyl, methoxy, hydroxy, nitro, amino, carboxyl and carboxymethyl substituents on ortho and para positions of the 11‐phenyl ring. All the spectra analyzed contain molecular ions and the principal fragmentation routes takes place either from the molecular or from (M+‐1) ion. There are, however, some deviations from the general fragmentation pattern in the case of 1,4‐dibenzodiazepin‐1‐ones with o‐nitro, o‐hydroxy and o‐carboxyl substituents caused by direct interactions of these groups with the benzodiazepine ring.