This work used Δ5-sterols and their degradation products to compare the efficiency of biotic and abiotic degradation processes in senescent Mediterranean marine (Posidonia oceanica) and terrestrial (Quercus ilex and Smilax aspera) angiosperms. Type II photosensitized oxidation processes appeared to be more efficient in P. oceanica than in Q. ilex and S. aspera. The low efficiency of these processes in senescent terrestrial angiosperms was attributed to: (i) the fast degradation of the sensitizer (chlorophyll) in these organisms and (ii) the relatively high on-ground temperatures observed in Mediterranean regions favoring the diffusion of singlet oxygen outside the membranes. Senescent leaves of P. oceanica contained the highest proportions of photochemically-produced 6-hydroperoxysterols, likely due to the presence of trace amounts of metal ions in seawater catalyzing selective homolytic cleavage of 5- and 7-hydroperoxysterols. Bacterial metabolites of sitosterol and its photooxidation products could be detected in senescent leaves of P. oceanica but not Q. ilex or S. aspera. These results confirmed that biotic and abiotic degradation processes may be intimately linked in the environment.