The slow magic angle spinning 2D exchange NMR expt. applied to semicryst. polyoxymethylene shows the existence of ultraslow mol. rotations in cryst. regions of the material. Simulation of the 2-dimensional spinning sideband spectra, which are characteristic for the mol. motion involved, shows that a rotation of the polyoxymethylene helical chains over 200 Deg is responsible for the obsd. spectra. The activation energy is 20 kcal/mol, in good agreement with values obtained for the a relaxation in dynamic mech. and dielec. studies.