The Diels–Alder dimer (11) of 1,3-dimethylcylcopenta[I]phenanthren-2-one (8) is conveniently prepared by dehydrochlorination of 1-chloro-1,3-dimethylcyclopenta[I]phenanthren-2-one (10). As judged from trapping experiments with N-phenylmaleimide, 1-phenyltriazoline-2,5-dione, and cyclopentadiene the dimer is in rapid equilibrium with the monomer (8) at 20 °C, but no colour accompanies dimer dissociation. endo-Addition of non-conjugated dienophiles [cyclopentene, cis-but-2-ene, and the cyclobutene (23)] is more important for (8) than for 2,5-dimethyl-3,4-diphenylcyclopentadienone (6), supporting the view that steric effects associated with non-coplanar phenyl groups in (6) impede endo-addition. Addition of (8) to tropone gives the exo-[6 + 4]π-adduct (21). Either thermolysis or photolysis of (8) affords the formal [4 + 4]π-dimer (32) of 1,3-dimethylcyclopenta[I]phenanthren-2-one. 1,3-Diethylcylopenta[I]phenanthren-2-one likewise exists as a dissociating [4 + 2]π-dimer (34), but 1,3-di-isopropylcyclopenta[I]phenanthren-2-one (35) is a sterically stabilised monomer which shows the same reactivity sequence towards dienophiles as observed for the dimer (11). The unusual n.m.r. spectra of the dimers (11) and (34) are tentatively rationalised in terms of a rapid Cope rearrangement.