at a top pressure which is from 0 10 to 10 0 bar higher than the top pressure of the column in which process stage ii is carried out the top stream from the trioxane purifying column is preferably fed to a further column in which the bottom stream drawn off is pure water this column is preferably designed with from 5 to 50 in particular with from 10 to 30 theoretical plates and is operated at a top pressure between 1 0 and 10 bar absolute preferably at a top pressure between 2 5 and 6 5 bar absolute the trioxane purifying column and or the column in which the bottom stream drawn off is pure water is preferably designed in such a way that the stripping section has from 25 to 100 preferably from 75 to 100 more preferably from 90 to 100 of the total number of theoretical plates of the column advantageously the side stream from the column in which process stage ii is carried out and or a further aqueous stream of the column in which pure water is obtained is fed and or the top stream from the column in which pure water is obtained is fed to the column in which process stage ii is carried out the further aqueous stream which is fed to the column in which pure trioxane is obtained preferably does not comprise any components extraneous to the process and preferably has a water content of at least 10 by weight in particular of at least 50 by weight advantageously instead of the trioxane purifying column and.