Temperature-programmed desorption and Auger spectroscopy were used to study the interaction of H 2 O and Cl 2 on an Fe polycrystalline surface under UHV conditions over the temperature range of 90-900 K. The TPD spectra of H 2 O on a clean Fe(poly) surface show the desorption of H 2 O from two states at 175 and 205 K. There is no recombinant H 2 O desorption from the Fe(poly), in contrast to the Fe(100) surface. The adsorption and dissociation of H 2 O is significantly affected by the presence of Cl at the Fe(poly) surface. Pre-exposure of Cl 2 results in dissociatively adsorbed Cl(Cl 2 ) at the surface. Cl 2 acts as a site blocker, preventing H 2 O dissociation. Cl 2 adsorption on an Fe(poly) surface pre-exposed to H 2 O at 90 K leads to a Cl 2 -H 2 O reaction which results in the formation of an OH-Cl complex. This complex is stabilized to temperatures above 400 K in UHV and results in recombinant H 2 O desorption over a broad temperature range. Desorption of an FeOCl complex is observed at 240 K regardless of dosing sequence, but only at low adsorbate pre-coverages. These data demonstrate that Cl 2 inhibits hydroxylation of the Fe(poly) surface. The low temperature volatilization of the Fe surface via Fe oxychloride formation does not directly involve H 2 O a , but can only occur after H 2 O dissociation.