The effect of Cu on the reduction behavior and surface properties of supported and unsupported Fe-based Fischer-Tropsch synthesis (FTS) catalysts was investigated using in situ X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) in combination with ex situ bulk characterization. During exposure to 0.4 mbar CO-H(2) above 180 degrees C, the reduction of CuO to Cu(0) marked the onset of the reduction of Fe(3)O(4) to alpha-Fe. The promotion effects of Cu are explained by a combination of spillover of H(2) and/or CO molecules from metallic Cu(0) nuclei to closely associated iron oxide species and textural promotion. XAS showed that in the supported catalyst, Cu(+) and Fe(2+) species were stabilized by SiO(2) and, as a result, Fe species were not reduced significantly beyond Fe(3)O(4) and Fe(2+), even after treatment at 350 degrees C. After the reduction treatment, XPS showed that the concentration of oxygen and carbon surface species was higher in the presence of Cu. Furthermore, it was observed that the unsupported, Cu-containing catalyst showed higher CO(2) concentration in the product gas stream during and after reduction and Fe surface species were slightly oxidized after prolonged exposure to CO-H(2). These observations suggest that, in addition to facilitating the reduction of the iron oxide phase, Cu also plays a direct role in altering the surface chemistry of Fe-based FTS catalysts.