[en] Reactive and nonreactive gas-surface interactions were studied by modulated molcular beam mass spectrometry on Pt(111) and stepped Pt(111) surfaces under ultra-high vacuum conditions. The angular distribution of helium atoms scattered from the stepped Pt(553) surface was measured as a function of the angle of incidence (polar, theta, and azimuthal, Phi). Rainbow scattering was observed indicating that the ordered array of steps and terraces exhibits a strongly periodic surface potential. In contrast, from the Pt(111) surface, the angular distribution of scattered helium atoms exhibits a sharp peak at the specular angle, characteristic of a smooth and well ordered surface. The interaction of hydrogen with platinum was investigated by studying the H₂-D₂ exchange reaction on the Pt(111) and the two stepped surfaces, Pt(553) and Pt(332). The production of HD was monitored. The mechanism of the H₂-D₂ exchange reaction was investigated on the Pt(111) and stepped Pt(332) surfaces. The dependence of the apparent reaction probability on surface temperature also indicates that the adsorption of hydrogen is an activated process on the Pt(111) surface, but not on the stepped Pt(332) surface. The energy barrier for hydrogen adsorption on Pt(111) deduced from this type of experiment is approx. 1.5 kcal/mole. On both surfaces, the mechanism of the recombination of adsorbed H and D atoms to form HD consists of a branched process at high crystal temperatures, with one of the branches connected in series with another reaction step at low temperatures. The process that isoperative over the entire temperature range studied, 25 to 800⁰C, has an activation energy and an apparent pre-exponential factor of E₁ = 13.0 +- 0.4 kcal/mole and A₁ = (8 +- 3) x 10⁴ sec^-1 for the Pt(332) surface and E₁ = 15.6 +- 0.5 kcal/mole and A₁ = (2.7 +- 1) x 10⁵ sec ^-1 for the Pt(111) surface