Abstract Ammonium jarosite sediment is a by-product of hydrometallurgical process used to extract zinc metal, which, which contains heavy metal ions and raises severe environmental concerns The transformation of jarosite sediment into high-value-added sulfide products through simple processing is a cost-effective and efficient strategy to overcome environmental and waste management issues. Herein, the influence of sulfur on thermal decomposition of ammonium jarosite is investigated in reducing atmosphere. The results reveal that the presence of sulfur promoted the decomposition of ammonium jarosite and szomolnokite and iron oxide phases have been observed after being heat treated at 300 °C. Moreover, after heat treatment at 700 °C, the decomposition of jarosite/sulfur mixture resulted in the formation of pyrrhotite phase, which can be used as a raw material for sulfuric acid production. Lastly, the activation energy of pyrrhotite formation has been estimated by using KAS equation and found to be 216.2 kJ/mol in reducing atmosphere.