Abstract Carbon tetrabromide adsorbs dissociatively on Fe(100). The sticking probability remains constant from zero to θ > 0.9, indicating the existence of a long lived, mobile pre-cursor state. The work function increases on adsorbing CBr 4, reachng a maximum value of Δø = 0.95 ± 0.02 eV at saturation. The diffraction pattern formed by room temperature adsorptionn is a (2 sin α′ × 2 sin α′) Rα′ structure. This is the same as the structure already observed for Br 2 adsorption on Fe(100). Heating the saturated surface causes the adsorbed carbon to diffuse into the bulk. On further heating (which brings about a decrease in bromine concentration), the surface behaves in exactly the same way as Br 2/Fe(100) under the same conditions. Adsorption of CBr 4 on the saturated surface, at room and elevated temperatures, after removal of carbon by heating, and adsorption of Br 2 on the saturated surface before and after removal of the carbon, have also been studied. The behaviour is shown to be consistent with the models used for Br 2 adsorption. The carbon from the dissociated molecule is assumed to lie deep in the four-fold sites while the larger bromine atoms sit high on the surface and dominate the surface structure. Comparisons are drawn between the CBr 4 and CCl 4 adsorption systems.