Kinetics of the solid state polymerization of disulphur dinitride into poly(sulphur nitride) was investigated using Fourier transform infrared spectroscopy in the temperature range 22 to 40 ° C. It was found that the polymerization is autocatalytic. The polymerization rate is strongly dependent on temperature and the overall activation energy of the polymerization in this temperature range was found to be 41 kcal mol−1. The polymerization is also strongly dependent on the crystal size, at 23 ° C the polymerization of needle-like crystals of about 5μm in width was complete in about 6 h but crystals of several millimetres in size reached conversion of only 40% in 6 days. It is concluded that the polymerization is much faster at the crystal surface than in its interior. Hence, it is suggested that the polymerization begins at the crystal surface and progresses towards the interior of the monomer crystal. The polymerization mechanism is discussed in terms of Baughman's theory of single-phase solid state polymerization; it is suggested that the autocatalytic character of S2N2polymerization is better described by the effect of the nearest neighbour interactions than by the effect of long range lattice st