Cobalt is extracted as a byproduct of oxidizing nickel and copper-cobalt ores during pyrometallurgical processing. Received mattes which contain up to 12 % mass of cobalt can be directly related to the Fe-Co-S system. Phase relations in the Fe-Co-S system were studied experimentally in Refs. [1-4]. Vogel and Hillner [1] reported liquidus projection and several vertical sections of the Fe-Co-S system. Three invariant reactions on the liquidus projection were detected [1,2] and then included to the scheil scheme reaction with some corrections [5]. Based on the literature data for the boundary binary systems, the Fe-Co-S was partially calculated thermodynamically [5,6]. Mainly isothermal sections at high temperatures (in the temperature range between 1100°C and 1350°C) were calculated. Nevertheless, the thermodynamic data have some contradictions with respect to the available literature data. Thus, the aim of the present work is an experimental reinvestigation of the phase equilibria in the Fe-Co-S system with the subsequent possibility of thermodynamic calculation of the entire system. For this, about 90 different compositions of the system were prepared by melting in an induction furnace followed by quenching in water. Samples were examined by X-ray diffraction. The microstructure characterization was carried out using SEM/EDX. Phase transformations temperatures and melting behavior of the samples were studied by DTA. Combination of the obtained results and mathematical treatment of these for the liquidus temperatures allowed visualizing phase transformations of the Fe-Co-S system as the liquidus and solidus projections.