A rock sample is subjected to a heating sequence in an inert atmosphere, the effluents resulting from this heating are oxidized, the hydrocarbon-based compounds, the CO, the CO2 and the SO2 released are measured, and a pyrolysis pyritic sulfur content is deduced therefrom. The residue resulting from the heating in an inert atmosphere is then heated in an oxidizing atmosphere and the CO and the CO2 released are measured. The pyritic sulfur content is determined at least from the pyrolysis pyritic sulfur content and from a parameter which is a function of the hydrogen content and of the oxygen content of the organic matter of the sample. It is also possible to determine the organic sulfur content from the pyritic sulfur content and from a measurement of the SO2 during the heating sequence in an oxidizing atmosphere.