Synthesis gas comprising a mixture of carbon monoxide and hydrogen is derived from fossil fuels and catalytically converted in a first reaction zone to a mixture of methanol and dimethyl ether which in turn is converted in a separate reaction zone in contact with a crystalline aluminosilicate zeolite catalyst having a silica to alumina ratio of at least about 12 and a constraint index of about 1 to 12, and preferably a crystal density in the hydrogen form of not substantially below about 1.6 grams per cubic centimeter to a product which is resolved into a high octane gasoline fraction, a light hydrocarbon gas fraction which may be liquefied and a hydrogen-rich gaseous by-product which is recycled to the conversion of fossil fuels to synthesis gas or may be otherwise used.