β-Diketones as well as β-dialdehydes react with 1,2-dianilinoethane to form 2,3-dihydro-1,4-diphenyl-1,4-diazepinium salts. Other NN′-disubstituted ethylenediamines react similarly. The NN′-dihydrodiphenyl-diazepinium salts are stable in acid but not in alkali. Substitution of the diazepinium cation by bromine has been effected.