The selective reduction of acetic acid to acetaldehyde on iron oxides has been studied in various pressure ranges. A good selectivity towards acetaldehyde could be achieved with molecular hydrogen continuously present in the gas phase and on a prereduced catalyst containing both a metal phase (zero-valent metal) and an oxidic phase. The pressure of acetic acid appears to influence the formation and the kind of by-products. At low pressures of acetic acid (0.015 mbar) ketene (CH2CO) is the most important (by-)product, while at high pressures (25 mbar) ketene has never been observed and acetone is the main (by-) product. Reaction mechanisms have been proposed for the selective reduction to acetaldehyde and for the formation of the (by-)products. Presumably the reduction to acetaldehyde occurs on the vacancy-rich oxidic layer, while hydrogen is dissociated on the zero-valent metal sites. The by-products are most likely formed via a common ketene-like intermediate.