Abstract: Methyl tertiary butyl ether is produced from a mixture of C.sub.4 hydrocarbons containing isobutylene and butadiene. The isobutylene is selectively reacted with a greater than stoichiometric amount of methanol at a temperature of 50 to 120 degrees C., in a homogeneous liquid phase and in the presence of a catalytic amount of sulfuric acid. The sulfuric acid is present in an amount less than 20% by weight based on the weight of the reaction mixture. The homogeneous liquid phase is maintained throughout the reaction whereby a reaction mixture containing methyl tertiary butyl ether is produced and undesired polymerization of the badadiene is avoided. The reaction mixture is cooled then reaction mixture which has been washed free of methanol, or separated unreacted C.sub.4 fraction which is isobutylene-free, is added. An upper phase containing hydrocarbons and methyl tertiary butyl ether and a part of the methanol and a lower phase containing methanol and 10 to 50% sulfuric acid is formed.