Sterol and alkenone compositions in suspended particle and surface sediment samples collected in the Northwestern Mediterranean Sea during the MEDFLUX program were used to evaluate the relative importance of biotic and abiotic degradation processes on marine organic matter. Alkenone concentrations decreased much more rapidly (~500 fold) between 5 and 800 m than Δ 5 -sterols (~100-fold) or POC (~100fold). The diverse functional groups attached to the stable tetracyclic carbon skeleton of Δ 5 -sterols appeared to be useful for estimating the relative effects of biotic vs. abiotic (photooxidation and autoxidation) degradation. Products of abiotic degradation predominated over products of biotic degradation in suspended particles in the NW Mediterranean. For alkenones, the U 37 K′ index increased from 0.43 to 0.55 with increasing water depth, and a good correlation between variations of U 37 K′ and concentrations of specific Δ 5 -sterol autoxidation products points to selective autoxidation of alkenones in suspended particles. Stereomutated alkenones (with cis double bonds) were detected in the surface sediment, allowing us to estimate that stereomutation resulted in a +0.05 increase in U 37 K′ . Therefore, abiotic degradation may be another factor effect on alkenone-derived paleothermometry.