The pyrolysis of 1,1,4,4-tetramethyltetralin-2,3-dione (I) or of 1,1,3,3-tetra-methylindan-2-one (11) at 800�/0.01-0.2 mm with short contact time gave a hydrocarbon pyrolysate containing naphthalene (c.60%), accompanied by smaller amounts of indene, 2- and 3-methylindenes, 1,2-dihydronaphthalene, benzofulvene, 1- and 2-methylnaphthalenes, dimethylnaphthalenes, and several unidentified minor components. Pyrolysates of similar composition were obtained from o-isopropyl-a-methylstyrene (111), 1,1,3-trimethylindane (IV), and 1,l-dimethyl-indene (V). Benzofulvene (VI) was partly converted into naphthalene at 800�. The last four compounds are suggested as possible intermediates in the conversion of (I) and (11) into naphthalene. The pyrolytic behaviour of these compounds correlates well with their 70-eV mass spectra; this apparent correlation is a more complex case than has previously been reported.