Method for the combined production of butanol and hydrogen

The invention relates to a process for the combined production of butanol and hydrogen from biomass, comprising the steps of fermenting biomass to obtain butanol in a first reaction mixture; removing the butanol and hydrogen from the first reaction mixture to obtain effluent; and using the effluent as a substrate in a second reaction mixture in a process using low substrate concentrations, in particular a hydrogen production process. Preferably, the process using low substrate concentrations is a hydrogen production process and at least part of the end products of the hydrogen production process is removed from the second reaction mixture for obtaining an effluent that comprises organic acid, which effluent is returned to the first reaction mixture.