Heating stoichiometric amounts of carbon and pure CaSO4, synthetic gypsum or phosphogypsum in a nitrogen atmosphere, results in the formation of CaS between 700 degrees C and 1100 degrees C. Different heating rates were used to investigate the reaction, and the amount of CaS formed depends on the heating rate used. A quantitative XRD method was used to determine the amounts of CaSO4, CaS, CaO and C in the samples. More CaS formed with increasing heating rate. Addition of 5% Fe2O3 and 5% ZnO as catalysts lowers the temperature range, as well as the activation energy of the reaction. The relationship between the activation energy values and degree of conversion (alpha) for the reaction between carbon and CaSO4 indicates that it is a complex reaction, and that simultaneous competitive reactions are taking place.