Because of their importance and significance not only in fundamental chemistry and physics but also in practical metallurgy and mineralogy, the structures and phase relations of pyrrhotite have been extensively studied using various in-situ high-temperature characterization techniques. The main conclusions are as follows: (1) It is experimentally confirmed that troilite FeS shows a first-order phase transition above 300°C for which the structure transforms into NiAs-type. The order of this transition is shown to be consistent with Landau theory. An intense DTA anomaly at 120°C was found to correspond to a structural transformation with no volume discontinuity and to arise from ferroelectric and magnetic spin flip transitions. For the non-stoichiometric iron sulfide, however, a first-order crystallographic transition, which occurred irreversibly at this temperature, leads to a distorted hexagonal structure instead of the simple NiAs-type. These findings have clarified the contradictions that appear in the literature concerning the magnetic, ferroelectric and crystallographic transitions in this material. (2) The formation of Kagome nets is found to be the principal ordering in the iron-deficient monsulfide. For Fe7S8, based on the TEM and XRD observations at elevated temperatures, the ABCD stacking with the monoclinic symmetry was found to transform into the ABC stacking (or mixed stacking) with trigonal symmetry at temperatures between 200°C and 250°C. The vacancies disorder within the iron layers at 315°C, at which temperature the magnetic Curie transition occurs. (3) Pyrrhotites with the composition from Fe11S12 to Fe9S10 can form the ideal commensurate NC superstructure (N = integer, C = c-axis length), but, in reality, mixed stacking with varying degrees of ordering often results. The special feature of these NC structures is the ABCD stacking of Kagome nets and antiferromagnetic ordering. When the stacking becomes disordered at temperatures between 150°C and 250°C, the pyrrhotite undergoes an anti-Curie transition ([lambda]-transition), and then transforms into another commensurate NC structure which is characterized by vacancies disordered within the layers.