A method to increase the molecular weight of wood mannans and xylans comprising aromatic moieties comprises the steps: a) obtaining mannan and/or xylan from wood, b) subjecting the mannan/xylan to oxidizing conditions to convert at least a fraction of the aromatic groups into radicals and reacting said radicals with each other to obtain complexes of mannan/xylan with increased molecular weight. The method is easy and cost efficient and enables to upgrade small wood mannans and xylans that are important in the pulp and paper industry into larger mannans and xylans and more defined structures which makes their isolation easier and application areas broader. Another advantage is that separation of mannans and xylans from mannans and xylans with bound aromatic groups is provided. The wood mannans and xylans with increased molecular weight can be used for the manufacture of barrier films in packages, for instance for oxygen, liquids and gas.