Crystalline monomers of acrylamide (AAm) and methacrylamide (MAAm) could be polymerized in the presence of chlorine gas under ultraviolet (uv) irradiation, the shape of the solid crystals remaining unchanged during the polymerization. The yield of polymer increased with increasing amounts of chlorine. It was deduced that chlorine atoms formed by the decomposition of chlorine molecules on the monomer crystals with uv irradiation initiate the polymerization, which proceeds from the surface to the inner part of the crystals. One apparent characteristic of the catalytic solid-state polymerization is that the molecular weight of the polymer increases as polymerization temperature is raised.