Zirconium tetrachloride efficiently catalyzes the cyclocondensation reaction of o-phenylenediamine and a ketone in refluxing 1,2-dichloroethane to afford the corresponding 2,3-dihydro-1H-1,5-benzodiazepine in high yield. The formation of specific regioisomers and their structural elucidation are reported for the first time.Key words: zirconium tetrachloride, o-phenylenediamines, ketones, 1,5-benzodiazepines, 1H NMR, regioisomers.