Abstract: The synthesis of tetrachlorophthalic (TCPA) was studied by catalytic chlorination of phthalic anhydride (PAA) in the medium of chlorosulfonic acid using iron and iodine as catalyst. Qualitative analysis of chloro-products and its relative percentage of chloroproducts during different chlorination reaction stages (monochloro-phthalic anhydride, dichloro-phthalic anhydride, trichlorophthalic anhydride and isomerides) was analysed by means of GC-MS. Optimum process condition for promoting of product yield was found.