In a process for the gasification of carbonaceous substances, exothermic and endothermic gasification agents are injected into the reaction area through at least one nozzle comprising at least two coaxial pipes, the outer pipe terminating short of the end of the inner pipe, at a distance such that the end face of the outer pipe is in a region of the pressurized interior of the reactor, in which the temperature is below the melting point of the ash of the solid particles to be gasified. The speed of flow of the endothermic agent out of the annular gap between the pipes is substantially higher than the speed of flow of the exothermic agent out of the inner pipe and the flow of endothermic gasification agent tends to be constricted around the flow from the inner pipe. The outer edge of the inner pipe is a sharp, right-angled edge so that in the region thereof eddies are formed which protect the region from the penetration of particles of ash therein.