Temperature-programmed reduction (TPR) with a well-swept fixed-bed reactor operating at 15 MPa of hydrogen pressure has facilitated the speciation of the organic sulfur forms ire coals and petroleum source rocks through complete hydrodesulfurization with typically 75-80% of the sulfur being released as hydrogen sulfide with the remaining 20-25% occurring as primarily thiophenic compounds in the oils obtained in high yield. In this study, the technique has been applied to kerogens isolated from a series of unconsolidated Peru margin sediments, and for comparison, to type I and II-S kerogens (Goynuk oil shale and Monterey Fm, respectively) from immature organic-rich rocks. Visual resolution in the hydrogen sulfide TPR evolution profiles has been achieved between mono; and di/polysulfidic forms with the latter being reduced below 250 degrees C. Indeed, di/polysulfides account for high proportions of the reduced sulfur in surficial sediment samples collected from close to the top of the sediment. In contrast, monosulfides are the dominant sulfur form in the sample analyzed from deeper in the Peru margin sediment where thiophenes account for up to ca. 35% of the organic sulfur. This distribution is remarkably similar to those found for both the type I and II-S kerogens investigated, indicating that the major changes in organic sulfur forms occur during the early stages of diagenesis.