NOVELTY - Producing ethanol and butanol using two separate fermentation steps, comprises: (a) passing a biomass feedstock without pasteurization or sterilization to a first fermentation step; (b) subjecting the biomass to anaerobic fermentation using a mixture of bacterial strains maintained at conditions to convert the biomass to a product mainly comprising acetic acid and butyric acid; (c) treating the product stream to separate a solution comprising the acetic acid and butyric acid; and (d) fermenting the solution from step (c) in a second fermentation step using a solventogenic bacteria.    USE - Ethanol is useful as a fuel or chemical feed stock.    ADVANTAGE - The first fermentation step is relatively robust, simple to operate and maintain in continuous operation and flexible to feedstock changes, compared to the equivalent step of US5753474. The process ensures an economical separation of butanol at low concentration from fermentation broths. Even though the butanol concentration in the product stream from step (d) is below 1 wt.%, this can be economically recovered because it is recovered in conjunction with ethanol recovery. A stream having a concentration of butanol of about 40 wt.% (of the total stream) may be obtained without a significant increase in column duty.    DETAILED DESCRIPTION - Producing ethanol and butanol using two separate fermentation steps, comprises: (a) passing a biomass feedstock without pasteurization or sterilization to a first fermentation step; (b) subjecting the biomass feedstock to anaerobic fermentation using a mixture of bacterial strains maintained at conditions to convert the biomass to a product mainly comprising acetic acid and butyric acid; (c) treating the product stream to separate a solution comprising the acetic acid and butyric acid by: (i) separating a solution comprising the acetic acid and butyric acid from any residual solids and (ii) separating bacteria and/or pasteurizing or sterilizing the solution from the first fermentation step; and (d) fermenting the solution comprising the acetic acid and butyric acid from step (c) in a second fermentation step using a solventogenic bacteria.