Abstract: There is disclosed a process for purifying aqueous solutions of N-methyl-morpholine N-oxide (NMMO), such as spinning bath solutions incurred in the production of cellulose products, in which process the solutions to be purified are contacted with an anion exchanger and the purified solutions are separated from the anion exchanger. The process is carried out in one step and the anion exchanger, as functional groups, exclusively contains quaternary tetraalkylammonium groups of the formulae--CH.sub.2 --N.sup..sym. (CH.sub.3).sub.3 X.sup..crclbar. or --CH.sub.2 --N.sup..sym. [(CH.sub.3).sub.2 (CH.sub.2 OH)]X.sup..crclbar.wherein X.sup..crclbar. represents the anion of an inorganic or organic acid. After this, the anion exchanger is regenerated with an aqueous acidic solution.