The mechanism of the deoxygenation of fatty acids on transition-metal sulfides was determined on the basis of kinetic data obtained with fatty acids, their reaction intermediates (aldehyde and alcohol), and reactants of restricted reactivity (adamantanyl-substituted carboxylic acids). Deoxygenation on MoS2 proceeds exclusively via hydrogenolysis to aldehyde, followed by hydrogenation to the corresponding alcohol, consecutive dehydration to the olefin, and hydrogenation to the alkane. In contrast, the selectivity on Ni-MoS2 and on Ni3S2 substantially shifts toward carbon oxide elimination routes: i.e., direct production of Cn–1 olefins and alkanes. The carbon losses occur by decarbonylation of a ketene intermediate, which forms only on sites associated with Ni. The rate determining steps are the cleavage of the C–C bond and the removal of oxygen from the surface below and above, respectively, 2.5 MPa of H2. The different reaction pathways catalyzed by MoS2 and Ni-MoS2 are attributed to a preferred deproton...