13C nuclear magnetic resonance and vapor-phase chromatography have been used to investigate the conversions of methanol and ethanol to hydrocarbons on a synthetic zeolite of the type H-ZSM-5 as described by Mobil. Methanol is first dehydrated to dimethyl ether and ethylene. Then the reaction proceeds by two competitive paths: first, successive dehydration-methanolation steps to give branched aliphatics, and, second, polycondensation reactions leading to linear aliphatic and aromatic compounds. The basic mechanism is essentially the same for ethanol, with the major difference being that ethylene can also be formed by direct dehydration of ethanol. At variance to earlier proposals, a mechanism involving carbenium ions is proposed which accounts well for the high yield in branched hydrocarbons and the observation of methyl ethyl ether which is detected in the methanol conversion products.