A method for preparing a target chemical derivable from cellulosic biomass, involving detoxification of spent hydrolysate is provided. The method comprises the steps of providing cellulosic biomass, subjecting the cellulosic biomass to aqueous pretreatment, aqueous hydrolysis, and fermentation under conditions in which at least a part of the fermentable sugars are fermented into a primary target chemical, separating the primary target chemical from the fermented hydrolysate to provide a spent hydrolysate comprising inhibitory substances and detoxifying the spent hydrolysate by decreasing the concentration of at least one of the inhibitory substances using a detoxification biocatalyst selected from the group consisting of wild type, mutant and recombinant filamentous fungi and recirculating at least a part of the detoxified spent hydrolysate, optionally after further purification.