The products obtained by sublimation or radiation of trioxane by high energy radiations have been examined by X-ray diffraction analysis.It has been possible to demonstrate that they consist of twinned crystals of hexagonal polyoxymethylene. By using rotating and WEISSENBERG photographs, the geometric correlations between the elements of the twins have also been definitely established. ZUSAMMENFASSUNG:Durch Analyse von Rontgendiagrammen werden die Produkte uutersucht, die bei Sublimation oder Bestrahlung von Trioxan mit energiereiehen Strahlen entstehen.Man kann zeigen, daf3 diese Produkte aus Zwillingskristallen von hexagonalem Polyoxymethylen bestehen. Mit Hilfe von Drehkristall-und WEISSENBERG-Aufnahmen konnen die geometrischen Beziehungen zwischen den Zwillingselementen gesichert werden. c y-3.563 A and helicoidal chains of the 211 type12). 171 G. CARAZZOLO, S. LEGHISSA, and M. MAMMIBoth structure analyses of these POM modifications were carried out by using X-ray diagrams of powders or stretched fibers.On the contrary, X-ray diffraction patterns of the single-crystal type are shown by POM when it is formed either by repeated sublimations of trioxane (first obtained by HAMMICK and B O E R E E~~) in 1922) or from radiation of trioxane with high energy radiations (HER) (prepared in 1961 by HAYASHI et al.I4)).