Mechanisms of C-3-C-4 alcohol synthesis from syngas are elucidated over a Mg/Al mixed metal oxide (MMO) supported K/MoS2 catalyst via C-13(2)-ethanol and C-13(2)-ethylene co-feeds. K/bulk-MoS2 is used as a control catalyst to provide insight into the role of K/MoS2 and K/MoS2-MMO sites on higher alcohol formation pathways. Analysis of the products via C-13 NMR show preferential enrichment of terminal carbons in C-3-C-4 alcohols with both co-feeds, suggesting that CO insertion is the primary higher alcohol synthesis pathway, and that olefin carbonylation proceeds through the same pathway as alcohol formation. The observation of C-13(4)-1-butanol species during the CO hydrogenation reactions with C-13(2)-ethanol co-feeds provides conclusive evidence of ethanol self-coupling to 1-butanol as a secondary pathway. Additionally, acetate species are shown to be formed via an acyl precursor reacting with an alkoxy anion. Hydrogenation of an acetyl species (CH3CO*) to an ethoxy intermediate (C2H5O*) is shown to be largely irreversible under the reaction conditions employed, as no preferential enrichment is observed for the acetyl group in acetate species. Propionate species are shown to be formed via esterification of propionate with the corresponding alcohol, while isobutyl alcohol formation observed over both the K/bulk-MoS2 and the MMO supported catalysts occurs via methanol coupling with 1-propanol or 1-propanol derived species. (C) 2016 Elsevier B.V. All rights reserved.