Apparatus for effecting the pyrolytic treatment of solid carbonizable materials, such as coal, shredded scrap-tires, comminuted municipal waste, sawdust and wood shavings, and the like. The treatment takes place in a cylindrical, horizontally-disposed reactor vessel including a material conveying device which transports the carbonizable materials through the vessel as a moving bed. A heating chamber is arranged coaxially around the reactor vessel and is configured as an annulus of substantially uniform inner diameter and of decreasing outer dimension from its forward end adjacent the materials outlet end of the reaction vessel, to its rearward end, adjacent the materials inlet end of such vessel. The material passing through the reaction vessel is subject to an indirect heat transfer relationship with a burning air-fuel mixture spirally swirling within the heating chamber and moving in a direction generally counter-current to the material passing through the reaction vessel. The burning air-fuel mixture and combusted gases are progressively constricted and confined by the heating chamber configuration and leave the heating chamber via an exhaust gas exit conduit located at a low point in the heating zone. During its passage through the reaction zone the material to be carbonized is converted by pyrolysis, or high-temperature destructive distillation, into combustible gases, liquid hydrocarbons and solid carbonaceous residues.