A biomass pyrolysis process is provided in which biomass feedstock is mixed with a heat carrier. The heat carrier at least partly comprises char. The ratio by weight of biomass to char is in the range 1:1 to 1:20. The process may be carried out by in a screw/auger pyrolysis reactor in which the solid feedstock components are conveyed along the reactor by a first screw. A second screw conveys at least a portion of the solid products of the biomass pyrolysis back to a heat transfer medium input port. Thus, the heat transfer medium includes char from the biomass pyrolysis.