Radiation-induced solid state polymerization of acenaphthylene was studied by using Co-60-gamma-rays. The extent of polymerization at room temperature reached 68% when acenaphthylene was irradiated with a dose of 2.5 MGy. Prolonged exposure to the radiation decreased the molecular weight of the polymers. The post-polymerization of acenaphthylene was found to occur at relatively high temperatures close to the melting point of acenaphthylene. Absence of an effective termination step allows the enhancement of post-polymerization at higher temperatures after a limiting conversion is accomplished.