A process for the production of syn gas (hydrogen and carbon monoxide) by reaction at high temperature between low molecular weight hydrocarbons, steam and oxygen in an impurities-containing refractory lined reaction zone. The lined reaction zone is pretreated with steam, or with steam and a reducing gas, e.g. a mixture of steam and hydrogen to leach out, react with, and transport the impurities, i.e., the reaction products of silica, or silica plus phosphorus or sulfur, or silica plus phosphorus and sulfur, from the reaction zone. Steam alone may be used to leach out, convert the silica to gaseous silicic acid, and remove same from the reaction zone; and then the hydrocarbons, steam and oxygen feeds are introduced into the reaction zone to produce syn gas.