Heating 1-trideuteriomethylbenzocyclobutenol 7 in benzene-d 6 at 160 °C gave 2-monodeuteriomethyl- and 2-methylacetophenone 9 and 10 in a ratio of 96:4. Thermolysis of 7 in nonpolar solvents (hexane, toluene, mesitylene) gave similar results. On the contrary, heating 7 in polar solvents (ethanol, acetonitrile, chloroform) or in benzene-d 6 in the presence of proton source (PhCO2H) gave 10 as the major product. However, heating a mixture of 7 and N-phenylmaleimide at 160 °C in benzene-d 6 or acetonitrile-d 3 gave adduct 12 of N-phenylmaleimide and the dienol generated by ring opening of 7 almost quantitatively. These results indicate that 1-methylbenzocyclobutenol undergoes selective thermal opening to the E-dienol. The resulting E-dienol isomerizes to 2-methylacetophenone by 1,5-sigmatropic shift of hydrogen from the methyl group in nonpolar solvent. In polar solvent, the E-dienol isomerizes to 2-methylacetophenone by both intra- and intermolecular processes. The k H/k D value for isomerization was 1.13. Since this relatively low value is a secondary kinetic effect, overall reaction is governed by the ring opening step. The selective opening to the E-dienol was supported by calculation.