The acid-catalyzed photoreaction of 6-chloro-1,3-dimethyluracil (6-CIDMU) with various polycyclic arenes was investigated: UV-Irradiation of a solution of 6-CIDMU with phenanthrene (1a), 9-cyanophenanthrene (1b) and pyrene (1f) in cyclohexane effected 1,2-cycloaddition to furnish the corresponding cyclobutapyrimidines as the predominant adducts, while the substitution reaction with acenaphthene (1e) and chrysene (1g) proceeded to give the corresponding 6-aryl-1,3-dimethyluracils.