The electrochemical behaviour at the mecury electrode of 1, 4-benzodiazepines and 1, 4-benzodiazépines-2-one was investigated. Medazepam, diazepam and desmethyldiazepam were reducibles according to a 2 F.mol −1 process. The plot of half-wave potentials vs pH is in agreement with the results of the spectrophotometric uv analysis: the more basic atom of the diazepine ring was that of the imine bond (C 5 = N 4). The introduction of an OH substituent on the carbon 3 resulted in 4 F.mol −1 process (oxazepam and lorazepam). The 2 F.mol −1 reduced intermediate was isolated. It underwent an acid-catalysed dehydration reaction and yielded in slightly basic solution a dimeric insoluble product. For oxazepam, the dimerisation initiated a disproportionation reaction.