The formation of decomposition products of macromolecules was found upon friction of polyoxymethylene against polyoxymethylene in the vacuum of mass-spectrometer . Their composition is shown to be different from that of the products of thermal decomposition of polyoxymethylene. It is shown that the formation of decomposition products is accompanied by the transition of the friction force from low to high level. The effect has been interpreted by the mechanical rupture of macromolecules upon friction. Quantitative parameters of the process have been discussed. Keywords: friction, mass-spectrometry, polymer, vacuum.