Reactions of 7,9-dimethyl-, 7,9-diethyl-, and 7,9-di-n-propyl-8H-cyclopenta[a]acenaphthylen-8-oneswith dimethyl 1-cyclobutene-l,2-dicarboxylate afforded the corresponding decarbonylated 1:1 adducts, while a similar addition of 7,9-diisopropyl-8H-cyclopenta[a]acenaphthylen-8-one onto the cyclobutene gave a 1:1 adduct. The photoaromatization and valence tautomerization of these adducts were examined.