An improved method for the synthesis of 7-chloro-5-(2-chlorophenyl)-2-(2-dimethylaminoethylthio)-3H-1, 4-benzodiazepine (IIa) and related compounds is described. IIa was obtained in 75.1% yield by the reaction of the 1, 4-benzodiazepin-2-one (Ia) with 2-dimethylaminoethanethiol in the presence of titanium tetracbloride ; a small amount of 7-chloro-5-(2-chlorophenyl)-2-dimethylamino-3H-1, 4-benzodiazepine (III) was obtained as a byproduct. A mechanistic interpretation of the formation of III is presented.