Chemistry Department, Faculty of Science, Suez Canal University, Ismailia, Egypt Manuscript received 10 April 1995, accepted 6 July 1995 1-Chlorobenzoi dphthalazinone (2) and 1,4-dichlorobenzoi [g]phthalazine (3) have been synthesised by the reaction of 1,2,3,4- tetrahydrophthalazine-2,4-dione (1) with phosphorus oxychloride. The chlorine atoms can be replaced by nucleophilic reagents such as morpholine, cyanide, amines, alkoxide, azide and hydrazine to give the corresponding products. Treating of hydrazino derivative (4) with formic acid afforded triazolophthalazinone derivative (5), while treating with acetic acid and/or acetic anhydride it gave triazoloph­thalazine derivatives, 6 and 7 respectively. Reaction of 4 with nitrous acid afforded azido derivative (8) which was also obtained from the reaction of 2 with sodium azide.