Lignocellulosic plant biomass is a renewable and abundant source for the production of bio-based fuels, chemicals and chemical building blocks. Efficient fractionation and conversion of these feedstocks are an essential step in the valorization of the cellulose, hemicellulose and lignin fractions. The use of a new two-stage alkaline oxidation (AlkOx) process has been investigated for the pretreatment of softwood in presence of sodium carbonate. Within this study, the use of commercial polymeric and ceramic ultrafiltration membranes for the purification and concentration of lignin from the spent liquor of the AlkOx process has been evaluated enabling further valorization thereof. Higher permeation fluxes were observed, ranging from 30 to 139 L/m 2 h, depending on the hydrophilicity, pore size and structure/chemistry of the membrane. High lignin retentions have been obtained for all membranes. Diafiltration of the spent liquor using the ESP04 membrane enables the purification of the lignin fraction with an efficient removal of the impurities originating from both the lignocellulosic material and the pretreatment. The integration of both processes allows the co-valorization of the lignin fraction, besides the primary C5/C6 sugar fraction produced in the two-stage alkaline oxidation of softwood. Also some preliminary techno-economic calculations have been realized on the membrane separation process to asses the economic potential of this technology.