A process for the co-gasification of carbonaceous solids (coal, coke) and biomass in which the biomass material is pyrolyzed to provide a biomass pyrolysis oil and biomass char or coke which are then mixed with the carbonaceous solid to form a slurry. This slurry is then heated if necessary to achieve a viscosity which can be processed conveniently in the gasifier. The heat required for pyrolyzing the biomass can conveniently be obtained from the heat exchanger used to cool the hot synthesis gas product emerging from the gasifier.