Abstract: Methyl tert-butyl ether is prepared, and a stream of hydrocarbons containing isobutylene is freed therefrom, by reacting the isobutylene with methanol in a two stage process wherein, in the first stage, a stream of hydrocarbons containing isobutylene and methanol is fed to a primary reaction zone where the isobutylene and methanol partially react in the temperature range of from 50.degree. to 90.degree.C and at a pressure of from 10 to 30 atmospheres in the presence of a catalyst to form methyl tert-butyl ether which is separated by distillation from the residue of isobutylene and methanol and all other hydrocarbons in the feed stream, and, in a second stage, the residue of isobutylene and methanol and other hydrocarbons is fed to a secondary reaction zone where the residue of isobutylene and methanol react in the vapor phase, at a temperature in the range of 60.degree. to 100.degree.