The exchange reaction between H-2 and D-2 has been studied over Pt{100} as a function of surface temperature between 150 and 1100 K using molecular beam techniques. The reactive sticking probability, s(r), shows strong hysteresis over this temperature range due to adsorbate-induced restructuring of the surface from hex-R to (1X1) structures. A nonlinear (1X1) island growth power law, rate alpha(theta(H)(hex))(4. 0), plays a critical role in the dynamics of the exchange process. Hydrogen adsorption below similar to 250 K induces incomplete (1X1) island growth. As the temperature is ramped upwards desorption leaves a clean (1X1) surface at similar to 400 K, with a high s(r)(=0.21), but by similar to 450 K the surface is converted to a defect-containing hex surface; the defects are characterized by a higher exchange activity than on the hex surface. At 650 K the defects are annealed away, and the surface exhibits the relatively low reactivity [s(r)=0.06+(4X10(-5)) T-s] of the stable Pt{100}-hex-R phase. On cooling from 1100 to 150 K, the hex-R structure remains until similar to 250 K, when (1X1) island growth causes irreversible hydrogen adsorption to saturation, with a sharp decrease in s(r).A kinetic model incorporating these features gives an excellent description of the data.