This study represents an explanation for the behavior of coal pyrite subjected to a linear temperature increase up to 1000 degrees C in an inert (TGA) as well as reductive (AP-TPR) atmosphere. The two pyrite samples were extracted from coals of the Halemba and Jastrzebie mines in Poland. In order to understand the behavior of the most frequent occuring inorganic iron-sulfur compounds in coal under AP-TPR conditions, conversion products of pyrite, namely pyrrhotite (Fe10S11) and troilite (FeS), were examined as well. An influence of the particle size on the pyrite reduction was observed. As for the sample mass, only an effect on the troilite reduction was found. Similarity of the AP-TPR profiles of coal-derived pyrite and mineral pyrite indicates the analogous reduction course. A study of the iron sulfate as another conversion product of pyrite is also included in this paper. The complementary techniques used in this study, TGA and TGA-MS together with AP-TPR, have enabled us to formulate a mechanism behind the pyrite and sulfate chemistry under reducing experimental conditions.