The behaviour of 5,6,7,8-tetrabromo-4-(3,4-dimethylphenyl)-lH-2,3-benzoxazin-1-one towards phosphorus pentasulfide, active methylene compounds, 3,5-dimethylpyrazole under Friedel—Craft's conditions, Grignard reagents, o-phenylenediamine, and hydrazine hydrate has been investigated. Also, reactions of 2-amino-5,6,7,8-tetrabromo-4-(3,4-dimethylphenyl)-2H-phthalazin-1-one with aromatic aldehyde, acetic anhydride, and ethyl acetate are described.