Four catalysts, 1.5% Fe/SiO2, 3.0% Fe/SiO2, 5.7% Fe/carbon, and Fe2O3, exhibited different phase transformations when subjected to reduction and reaction conditions. After reduction at 673 K in H-2 for 16 h, 31% of the iron in 1.5% Fe/SiO2 was in superparamagnetic Fe-0 particles with the remainder in Fe2+ species, while 3.0% Fe/SiO2. contained 91% of its iron as alpha-Fe-0 with the remainder as superparamagnetic Fe2+ species. The iron in 5.7% Fe/C existed mostly as alpha-Fe-0 (63%) with the balance present as Fe2+ species, which were mostly superparamagnetic, and the Fe2O3 was completely reduced to alpha-Fe-0. After 8 It on stream, only Fe2+ and Fe3+ species were detected in 1.5% Fe/SiO2, which gave only decomposition products; the 5.7% Fe/C catalyst contained alpha-Fe, Fe2+, Fe3+, and theta-Fe3C phases and had completely deactivated, and the two active, stable catalysts-3.0% Fe/SiO2 and Fe2O3-showed the presence of both alpha-Fe and FeO phases under steady-state reaction conditions. It is proposed that the former phase provides sites to activate H-2, and the latter provides different sites to adsorb and activate acetic acid by producing a reactive acetate intermediate. DRIFTS, coupled with TPD and TPR experiments, revealed that surface acetate species are formed during acetic acid adsorption at 300 K on iron surfaces and they appear to be an active intermediate at the reaction temperatures used here. Reaction of this surface acetate with H atoms via a Langmuir-Hinshelwood-type mechanism is proposed to be the principal reaction pathway for acetic acid reduction to acetaldehyde.