A process for the production of alcohol from a cellulosic or lignocellulosic substrate in which furfural is produced and which comprises at least the following steps: • a first step for alkaline chemical pre-treatment (step A) of said substrate, comprising a step A1 for heating in the presence of an alkaline chemical reagent, at the end of which at least one stream comprising at least said chemical reagent and the pre-treated substrate are obtained, and optionally a step A2 for washing said pre-treated substrate, at the end of which the pre-treated washed substrate and at least one stream containing spent washing water are obtained; then • a step for bringing the pre-treated and optionally washed substrate to a pH in the range 4.5 to 5.5, coupled with a step for enzymatic hydrolysis (step B1) of said pre-treated substrate using cellulolytic and/or hemicelluloytic enzymes; • a step (step B2) for alcoholic fermentation of the hydrolysate obtained with an alcohol-producing microorganism, at the end of which a fermentation mash is obtained comprising material in suspension and a liquid phase comprising an alcohol and vinasses; then • a step (step C) for separation/purification, at the end of which purified alcohol, clarified vinasses and an insoluble residue are obtained; and • a step (step D) for acid conversion of at least one of the liquids obtained during one of the preceding steps and containing dissolved pentoses, a portion of said pentoses being converted into furfural; and • a step for mixing furfural obtained during the acid conversion step with the pre-treated substrate optionally washed during the pH adjusting step, the quantity of furfural mixed in being in the range 1 to 5 g/L with respect to the total quantity of liquid employed in the enzymatic hydrolysis step and/or the alcoholic fermentation step. • a first step for alkaline chemical pre-treatment (step A) of said substrate, comprising a step A1 for heating in the presence of an alkaline chemical reagent, at the end of which at least one stream comprising at least said chemical reagent and the pre-treated substrate are obtained, and optionally a step A2 for washing said pre-treated substrate, at the end of which the pre-treated washed substrate and at least one stream containing spent washing water are obtained; then • a step for bringing the pre-treated and optionally washed substrate to a pH in the range 4.5 to 5.5, coupled with a step for enzymatic hydrolysis (step B1) of said pre-treated substrate using cellulolytic and/or hemicelluloytic enzymes; • a step (step B2) for alcoholic fermentation of the hydrolysate obtained with an alcohol-producing microorganism, at the end of which a fermentation mash is obtained comprising material in suspension and a liquid phase comprising an alcohol and vinasses; then • a step (step C) for separation/purification, at the end of which purified alcohol, clarified vinasses and an insoluble residue are obtained; and • a step (step D) for acid conversion of at least one of the liquids obtained during one of the preceding steps and containing dissolved pentoses, a portion of said pentoses being converted into furfural; and • a step for mixing furfural obtained during the acid conversion step with the pre-treated substrate optionally washed during the pH adjusting step, the quantity of furfural mixed in being in the range 1 to 5 g/L with respect to the total quantity of liquid employed in the enzymatic hydrolysis step and/or the alcoholic fermentation step.