Mass spectrometric behaviour of 2,2a,3,4-tetrahydro-4-methyl-2a-phenyl-2-(thiophen-2-yl)-1H-azeto[2,1-d][1,5]benzothiazepin-1-one-5-oxide and 2,3-dihydro-2,4-diphenyl-1,5-benzothiazepine-1-oxide/-1,1-dioxide have been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. The monooxide derivatives showed a tendency to eliminate an alkene or an oxygen atom. 1H-Azeto[2,1-d][1,5]benzothiazepin-1-one-5-oxide could also eliminate the thiophen-2-ylketene molecule via a reverse [2+2] cycloaddition. 2,3-Dihydro-2,4-diphenyl-1,5-benzothiazepine-1-oxide/-1,1-dioxide could eliminate SO_2 or SO, respectively. The structure of 2,2a,3,4-tetrahydro-4-methyl-2a-phenyl-2-(thiophen-2-yl)-1H-azeto[2,1-d][1,5]benzothiazepin-1-one-5-oxide was identified on the basis of its fragmentation. The identification was supported by the fragmentations of model compound, 2,3-dihydro-2,4-diphenyl-1,5-benzothiazepine-1-oxide/-1,1-dioxide. Mass spectrometry behavior of 2,2a, 3,4-tetrahydro-4- methyl-2a-phenyl-2- (thiophen-2- yl) -1H-azeto [2,1-d] [1,5] benzothiazepin-1 -one-5-oxide and 2,3-dihydro-2,4-diphenyl-1,5-benzothiazepine-1-oxide / -1,1-dioxide have been studied with the aid of mass-analyzed ion kinetic energy spectrometry and 1 mass spectrometry under electron impact ionization. The monooxide derivatives showed a tendency to eliminate an alkene or an oxygen atom. 1H-Azeto [2,1-d] [1,5] benzothiazepin-1-one-5-oxide could also eliminate The thiophen-2-ylketene molecule via a reverse [2 + 2] cycloaddition. 2,3-Dihydro-2,4-diphenyl-1,5-benzothiazepine-1-oxide / -1,1-dioxide could eliminate SO_2 or SO , respectively. The structure of 2,2a, 3,4-tetrahydro-4-methyl-2a-phenyl-2- (thiophen-2-yl) -1H-azeto [2,1-d] [1,5] benzothiazepin -1-one-5-oxide was identified on the basis of its fragmentation. The identification was supported by the fragmentations of model compound, 2,3-dihydro-2,4-diphenyl-1,5-benzothiazepine-1-oxide / -1,1-dioxide.