The influence of pre-treatment in a H-2/H2O atmosphere and of the H-2 pressure on the reduction behaviour of an unpromoted and potassium-promoted, zirconia-supported iron-based catalyst were studied. The initial dispersion of both freshly prepared catalysts is very high.These freshly prepared catalysts still contain carbon which originates from the citrate complex used in the preparation procedure. The largest amount of carbon is encountered on the potassium-promoted catalyst. During reduction at relatively low temperature and at ambient pressure, it is observed, that this initially present carbon is removed by the pre-treatment in a H-2/H2O atmosphere. Another method for removing the initially present carbon from the catalysts is a reduction treatment in H, at or above a pressure of 1.8 bar.It is further found that both treatments for removing carbon result in the formation of divalent iron already at relatively low temperatures. This iron species reacts with the support to a mixed oxide which stabilizes the high dispersion of the metallic iron particles formed upon reduction at higher temperatures. The divalent iron is easily removed out of the mixed oxide by oxidation at room temperature. (C) 2003 Elsevier B.V. All rights reserved.