During carbon monoxide hydrogenation on a natural iron catalyst, two different behaviors were observed when the operating pressure was changed from 3.5 to 20.4 bar at 513 K and inlet H-2/CO of 2.0. A gradual increase in the catalyst activity to a maximum followed by a gradual loss in activity was observed at lower operating pressure. The surface composition (XPS) for the used catalyst is mainly alpha-Fe, alpha-SiO2, and a great amount of surface carbon (mainly graphitic carbon). After hydrogen reduction treatment or Ar ion etching iron carbide species appears on the surface. Aliphatic and polyaromatic species were observed by fourier transform infrared. On increasing the operating pressure up to 20.4 bar, a dramatic increase in carbon monoxide conversion (four times higher) under steady state conditions was observed. The bulk composition of the catalyst as determined by Mossbauer spectroscopy is Fe3O4, (50.0%), alpha-Fe (31.1%) and chi-Fe5C2 (18.9%) in correspondence to a gas phase constituted mainly by alkanes, Thus, alpha-Fe and Fe3O4 appears to be correlated with the catalytic activity whereas the deactivation process appears to be caused mainly by the formation of graphitic carbon on the catalyst surface. The formation of such deposit is inhibited at higher operating pressure,