Products and kinetic and mechanistic aspects of the title reaction were investigated by time-resolved X-ray powder diffraction combined with Rietveld refinement. The polymerization of S2N2 at 40 degrees C was monitored with a time resolution better than one diffraction pattern per minute until completion. The reaction product comprised a mixture of monoclinic beta-(SN)(x) (90%) and an additional orthorhombic phase, identified as alpha-(SN)(x) (about 10%). The inversely proportional conversion/formation vs time curves for educt and products indicate that the reaction involves only crystalline phases. The cell parameters of polymerizing S2N2 varied noticeably with progressing conversion, whereas the cell volume remained unchanged up to a monomer conversion of 50%, Refinement of atomic coordinates for S and N indicated retention of the reactant geometry, It is concluded that the polymerization of S2N2 to (SN)(x) is of nondiffusive nature and involves a martensitic transition.