Abstract: A method of isolating trioxane by distillative separation at atmospheric pressure without extraction steps from an aqueous trioxane solution having a trioxane concentration which does not exceed that concentration, which has an constant boiling azeotropic mixture obtained at a simple distillation of said aqueous trioxane solution at normal pressure, includes heating the aqueous trioxane solution, which may also contain formaldehyde and acid, in an evaporator to form a trioxane and water vapor-containing steam mixture substantially of the azeotrope boiling temperature (about 92.degree. C. for a solution containing only water and trioxane); feeding an inert gas stream into either the evaporator or a partial condenser connected directly to the evaporator; leading the steam mixture in a first stage together with the inert gas stream through the partial condensing means, wherein the steam mixture together with the inert gas stream is cooled to a temperature of 58.degree. to 64.degree. C.