The invention relates to an improved process for separating lignin and monomeric sugars from a liquor comprising lignin and monomeric sugars in a solvent mixture of water and at least one organic solvent, which employs membrane filtration techniques such as nanofiltration and selective water removal, preferably bypermeation through a membrane which is selective for water molecules. The invention further relates to a modular system for executing the process according to the invention. The process and system according to the invention are particularly suitable to be incorporated with pre-treatment of lignocellulosic biomass, in particular by organosolv fractionation or solvolysis.