The mass spectrometric behaviour of seven 1,3a,5-trisubstituted 3a,4,5,11-tetrahydro- 6H-1,2,4-oxadiazolo[4,5-a][1,5]benzodiazepines has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and exact mass measurements using fast-atom bombardment ionization. All compounds show a tendency to eliminate a neutral substituted or unsubstituted styrene from the diazepine ring to yield 1,2,4-oxadiazolo[4,5-a][1,3]benzimidazole ions, and further undergo reverse 1,3-dipolar cycloadditions to give benzimidazole ions. The formation of stable conjugated fused tetracyclic systems, substituted 1,2,4-oxadiazolo[4,5-f]phenanthridine ions, was also observed. Copyright © 1999 John Wiley & Sons, Ltd.