The invention relates to a process of separating compounds with a small molar mass from compounds having a molar mass less than 1.9 times that of the compounds with a small molar mass using nanofiltration. The compounds with a small molar mass have a typical molar mass less than 250 g/mol. In one embodiment of the invention, pentose sugars are separated from hexose sugars. The invention can be applied to the recovery of xylose from spent liquors and to the recovery of betaine from sugar beet pulp extracts, for example.