A preparative method for the selective synthesis of 1-methylcyclobutene from methylenecyclobutane (MCB) was proposed. The basic features of heterogeneous catalytic isomerization of MCB on thermally activated aluminum oxide were studied. It was shown that the catalytic activity of alumina depended on its pretreatment temperature. The maximum isomerizing activity of alumina was observed after its successive activation at 850 degrees C in air and nitrogen streams for 1 h in each. The conversion of MCB into 1-methylcyclobutene was found to reach its equilibrium value (85%) at an isomerization temperature of 25 degrees C.