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In order to investigate the formation of organosulphur compounds during the very early stage of diagenesis, experiments were designed to simulate sulphur incorporation from H2S, or elemental sulphur, into recently deposited natural sedimentary organic matter (cyanobacterial mats from salt ponds in Baja California) and performed under relatively mild reaction conditions (40°C, 2 to 30 days, hydrous and anhydrous). The sulphur-containing moieties in the reaction products were examined by pyrolysis-gas chromatography (GC) using a GC equipped with a flame photometric detector (FPD) and a flame ionization detector (FID), and pyrolysis-gas chromatography/mass spectrometry (GC/MS). Sulphur incorporation from H2S into recently deposited cyanobacterial mats occurred fairly readily, but elemental sulphur does not react as readily with the cyanobacterial mats. Sulphur incorporation into humic acids occurred more rapidly than into proto-kerogens. Sulphur incorporation into lipid fractions was not observed. Two major sulphur incorporation stages are proposed, with the formation of thiols by reaction between H2S and active carbonyl groups considered to be the most likely mechanism of sulphur incorporation in the first stage. The second stage of sulphur incorporation is proposed to be related to the reaction between H2S and proteinaceous organic matter.