A derivatograph study has been carried out for the thermal decomposition of ferrous sulphate monohydrate in an oxidizing gaseous atmosphere. Based on the results from the derivatographic, thermogravimetric, and X-ray phase analyses, and from Mossbauer spectroscopy, a two-stage process for oxidizing Fe2+ to Fe3+ is suggested with formation of intermediate products, such as FeOHSO4 and Fe2O(SO4)(2). A new mechanism is suggested for the thermal decomposition of FeSO4 . H2O in an oxidizing gaseous atmosphere, that takes into account the effect of the partial pressure of the gaseous components on the rate of certain parallel reactions.