In relation to the phototoxicity of 7-chloro-I,4-benzodiazepine-N-oxides the photostability of some of these N-oxides and the thermostability of their photoproducts, the oxaziridines, were studied. Rather than a consequence of a direct phototoxic effect by the N-oxides the ultimate toxic effect in the test system Salmonella typhimurium appeared to be caused by products formed during and after the irradiation. For chlordiazepoxide (CDZ) and its main metabolites in man desmethylchlordiazepoxide (DES) and demoxepam (DEM) the formation of an oxaziridine is indeed a crucial factor for the onset of the toxic effect. However, DEM oxaziridine (DEM OX.) being very thermolabile in protic medium forms non-toxic conjugates with the solvent. CDZ OX. and DES OX. are thermally converted into their N-oxide, although DES OX. partly decomposes into 6-chloro-4-phenylquinazoline-2-carboxaldehyde as well. This compound proved to be an important factor in the toxic action of DES after irradiation.