Reactions of (CH 3) nη AlCl 3− n ( n = 1, 2, 3) with propylene oxide have been studied. At room temperature, donor-acceptor complexes are formed; at elevated temperatures, they enter into further reactions. With trimethylaluminium, addition occurs exclusively, resulting in opening of the epoxide ring chiefly at the more-substituted carbon atom and, after hydrolysis, 2-methylpropanol is the major product. With methylaluminium dichloride, which is a more powerful Lewis acid than trimethylaluminium and gives rise to enhanced mobility of hydrogen atoms attached to the epoxide ring carbon atoms in the complex molecule, elimination of methane occurs. The ring becomes opened primarily at the more-substituted carbon atom. With dimethylaluminium chloride, both reactions take place. Mechanisms for methane elimination and consecutive reactions which yield C 4 hydrocarbons and other products are discussed.