AbstractThe non‐oxidative decomposition of heated sodium dithionite was shown by thermal and i.r. spectroscopic analyses to proceed by two routes depending on sample configuration. The previously postulated reaction\documentclass{article}\pagestyle{empty}\begin{document}$$ 2{\rm Na}_2 {\rm S}_2 {\rm O}_4 \to {\rm Na}_2 {\rm S}_2 {\rm O}_3 + {\rm Na}_2 {\rm SO}_3 + {\rm SO}_2 $$\end{document}predominated when the heaped substance was heated at 16 °C/min giving a sudden exotherm of 47 kJ mol−1at 205 °C. In other conditions, particularly when heated in thin layers, two exotherms were found. The major exotherm at 210 °C yielded 63 kJ mol−1the main products being thiosulphate, trithionate, sulphate and sulphite. A subsequent exotherm at 250 °C appeared to arise from phase changes in this material, coincident with the endothermic decomposition of sodium tr