Phthalazine, also called benzo-orthodiazine or benzopyridazine, is a heterocyclic organic compound with the molecular formula C8H6N2.

Phthalazine can be obtained by the condensation of ω-tetrabromorthoxylene with hydrazine, or by the reduction of chlorphthalazine with phosphorus and hydroiodic acid.

[4] It possesses basic properties and forms addition products with alkyl iodides.

[4] Upon oxidation with alkaline potassium permanganate it yields pyridazine dicarboxylic acid.

The keto-hydro derivative phthalazone (C8H6ON2), is obtained by condensing hydrazine with orthophthalaldehydoacid[citation needed].