The vapor-phase reduction of acetic acid by H-2 over both supported and unsupported iron was studied. Typical Fe catalysts increased in activity over a 4- to 5-h period to reach steady state whereas the Fe/carbon catalyst deactivated completely during this initial time on stream. The best catalysts gave selectivities to acetaldehyde between 95 and 100% at low conversions (<12%), but selectivity dropped to similar to80% as conversions approached 40%. This parameter was enhanced as the H-2 pressure increased, but was almost independent of the acetic acid partial pressure. The turnover frequency for acetic acid disappearance to reduction products increased markedly from 0.003 to 0.058 s(-1) and the apparent activation energy decreased from about 27 to 16 kcal/mol as the Fe crystallite size increased from 10 to 4000 nm. Highly dispersed 1-nm crystallites on SiO2 exhibited no hydrogenation activity and gave only decomposition products. Acetic acid reduction over Fe is described well by a Langmuir-Hinshelwood model invoking two types of sites, i.e., one set on metallic Fe atoms that adsorbs and activates H-2, and the other on an Fe oxide surface that adsorbs acetic acid to form an acetate species. The addition of the second H atom to form acetaldehyde appears to be the rate-determining step.