3,3-Dimethyl- and 3-methyl-1-(4′,4′-dimethyl-2′,6′-dioxocyclohex-1′-yl)-3,4-dihydroisoquinolines have been synthesized. Crystal and molecular structures of the 3,3-dimethyl derivative have been determined. In the crystalline state this compound exists as a tautomeric form where a hydrogen atom is located at the N atom of the dihydroisoquinoline fragment of the molecule. The tautomeric equilibrium does not shift noticeably in solutions, as shown by IR, UV, and NMR spectroscopy. © 1994 Plenum Publishing Corporation.