A tube furnace for the cracking of organic feed stock, e.g. naphtha to ethylene, comprises a chamber traversed by at least one conduit system for the organic feed stock. The conduit system in the region of the inlet comprises a plurality of pipes which are united at an intermediate location and communicate jointly with a single pipe forming the discharge portion of the duct system. Preferably the flow cross section of the duct system increases from the inlet to the outlet end. Burners are provided in the wall of the chamber.