In this study, the thermal behavior of polyacenaphthylene (PAcN) and partially modified polyacenaphthylene with Cl, Br, and H 2 PO 3 groups, both under air and nitrogen atmosphere, have been investigated. The influence of the molecular weight, the heating rate, and the introduced substituent have been studied. Thermogravimetric Analysis combined with Fourier Transform Infrared Spectroscopy was used to determine the thermal stabilities and the degradation mechanism. All polyacenaphthylene samples degrade through a depolymerisation process under a nitrogen atmosphere, whereas in air, elimination and thermo-oxidation reactions take place.