Abstract: In a process for the thermal cracking of hydrocarbons by the introduction of liquid petroleum feedstock in atomized form into a stream of hot combustion products formed by the combustion of fuel and oxidant in successive burner and mixing zone, constricting throat zone, a velocity acceleration diffuser zone and secondary cracking and reaction zone along the path of hot combustion product stream flow, an improvement is disclosed which comprises: effecting initial mixing, vaporization and cracking of said atomized liquid petroleum feedstock in said stream of hot combustion products in said burner and mixing zone maintained at subsonic velocity flow; effecting substantially complete mixing and vaporization in said constricting throat zone wherein said stream is maintained, at exit, at sonic velocity flow; passing said stream through a velocity acceleration diffuser zone maintained at supersonic velocity flow; passing said stream through a shock region produced by cross-sectional expansion of a diffuser zone; and