The gas-phase thermal isomerization of 1,1-dimethylcyclopropane at 420 degrees C gives 3-methyl-1-butane and 2-methyl-2-butene. The mixture of products formed from 1,1-dimethyl-2,2-d(2)-cyclopropane is consistent with conventional rationales; neither tert-butylcarbene nor isobutylcarbene is involved as a mechanistically significant intermediate. Both kinetic and deuterium labeling evidence suggests that heterogeneous wall-catalyzed processes afford secondary products, including 2-methyl-1-butene.