Previous work on the interaction of hydrogen with Pt(111) was extended to a stepped Pt(S)-9 (111) × (111) surface in order to elucidate the role of structural imperfections as “active sites” in surface reactions. Experiments were performed by means of LEED, AES, ELS, thermal desorption and work function techniques which also served for characterization of the properties of the clean surfaces. The saturation coverage with dissociatively adsorbed hydrogen at 120 K was found to be near unity. The LEED pattern showed the appearance of additional weak streaks which were interpreted as being caused by one-dimensional order of those hydrogen atoms which are located in the vicinity of the steps. At low coverages the adsorption energy was found to increase up to about 12 kcal/mole whereas the corresponding value of the flat surface is 9.5 kcal mole . Above θ ≈ 0.3 the E ad versus θ-curves were identical for both types of surfaces. Pronounced effects were observed with kinetic processes: The presence of steps increases the initial sticking coefficient by a factor of four to a value of 0.34, and the activity for the H 2 — d 2 exchange reaction is enhanced by an order of magnitude whereby however the apparent activation energy remains constant. With the stepped plane the work function at first increases up to 25 mV at θ = 0.25 and then decreases to a final value of −350 mV, whereas adsorption on an perfect (111) plane causes only a continuous decrease. A detailed inspection of the data leads to the proposal of a model in which two (slightly different) types of adsorbed hydrogen atoms are associated with the atomic steps. Although the variations of the metalhydrogen binding energies between sites at steps and at low index planes are only very small there may be significant implications on the kinetics of surface reactions.