Trimethylaluminium in hydrocarbon solvent at 100-200� gem-dimethylates various ketones (and some aldehydes) to alkanes. gem-Dimethylated alkanes prepared by this method include 1,l- dimethylindane, 2,2-dicyclopropylpropane, 2,2-di-p-tolylpropane, 1-t-butyl-2,5-dichlorobenzene, 1,l-dimethylcycloheptane, 9,9-dimethyifluorene, 2,3-bis(p-methoxyphenyl)-2-methylpentane, 2,3- bis(p-methoxyphenyl)-2,4-dimethylpentane and 3,3-dimethylcholestane. Xanthenone and N-methyl-acridone are gem-dimethylated particularly easily. Anthrone, however, gives 9-methylanthracene.