The invention relates to a method for isolating xylan from plant material. In the method, cellulose fibres which contain xylan are used, advantageously cellulose fibres from hardwood; and xylan is extracted (20) from said cellulose fibres by means of sodium hydroxide for forming an extract solution (17) that contains xylan; cellulose fibres are removed from said mixture that contains cellulose fibres and extract solution (17), for isolating said extract solution from said mixture; carbon dioxide (14) is added to said extract solution (17) for isolating xylan, wherein liquid brightener (18) and precipitated xylan (12) are formed; calcium hydroxide (16) is added to said isolated brightener (18), for forming precipitated calcium carbonate and liquid sodium hydroxide (15); and sodium hydroxide is removed from said mixture that contains sodium hydroxide and precipitated calcium carbonate, for isolating calcium carbonate from said mixture. Furthermore, the invention relates to a system for isolating xylan from plant material, and to xylan, calcium carbonate, cellulose fibre and use of xylan.