Iron-based catalysts appeared to he very active and selective in the hydrogenation of acetic acid to acetaldehyde. The active and selective catalyst consists of a metallic and an oxidic phase. Probably, the metal is needed to activate hydrogen, and the oxide is needed to provide the reaction site for the selective hydrogenation. Catalyst pretreatment and reaction conditions must be carefully controlled : the catalyst must be prereduced and the hydrogen/acid ratio must be higher than four Only then are both iron-containing phases formed and kept stable during the reaction. The function of hydrogen is twofold : it must keep the catalyst in its active, partly reduced, form and it acts as reactant in the hydrogenation of acetic acid.