Abstract: The formation and accumulation of coke or other reaction by-products of olefins produced by the pyrolysis of hydrocarbons is prevented. This is accomplished by maintaining the transfer portion extending from the outlet port of the pyrolysis reaction tube to the multi-tubular quenching device at a temperature below 450.degree. C. This technique is especially applicable to the manufacture of olefins by the pyrolysis of hydrocarbons which are gaseous at room temperature under atmospheric pressure or liquid hydrocarbons having a volumetric average boiling point of below 90.degree. C., at temperatures ranging from 750.degree. to 900.degree. C., followed by quenching in a multi-tubular quenching apparatus. It is only necessary to cool the transfer portions to a temperature below 450.degree. C. in those areas where cracked gas is flowing at a flow rate below 50 kg/m.sup.2 per sec.