The conversion of methanol into hydrocarbons over ZSM-5 zeolites has been investigated using a gas-recirculation system. The conversion proceeds autocatalytically. The reaction rate is greatly enhanced by the addition of ethylene or cis-but-2-ene, indicating that the autocatalysis is caused by the reaction of olefin and methanol. Infrared studies have revealed that methoxyl groups (—OCD₃) are formed by the reaction of surface hydroxyl groups and methanol molecules (CD₃OH), and they decompose to reproduce hydroxyl groups (—OD), giving hydrocarbons. The reaction mechanism in which methyl carbonium ions attack the C—H bond of methanol or dimethyl ether molecules is proposed. Methyl carbonium ions are supposed to be released from extensively polarised methoxyl groups formed by the reaction of methanol molecules and strong Brönsted-acid sites. In fact, strongly acidic materials, Nafion H and heteropolyacids, were found effective for methanol conversion. A semiquantitative analysis of the autocatalytic phenomenon in methanol conversion is also presented.