The present invention is directed at a process for fractionating lignocellulosic biomass for the purpose of reducing processing costs, increasing delignification, reducing side-reactions, in particular reducing hemicelluloses degradation, improving cellulose hydrolysis and increasing the nativity of the obtained lignin, by performing organosolv fractionation of the biomass with a treatment liquid at a temperature below 170° C., wherein the treatment liquid comprises a non-hydroxylic organic solvent, in particular a ketone, water and some acid, and optionally subjecting the cellulose-enriched product stream to enzymatic hydrolysis.