We have explored the effect of alloying an unreactive metal, Sn, on the dynamics of D-2 dissociative chemisorption at Pt(111). By comparing D-2 sticking and recombinative desorption on Pt(111) with that on the ordered p(2 X 2) Sn/Pt(111) and (root 3 X root 3)R30 degrees Sn/Pt(111) surface alloys, we examine the influence of the local surface composition on reactivity. The energy dependence of D-2 sticking S(E) has been measured for all three surfaces using a hyperthermal beam. We find that the activation barrier for dissociative chemisorption is low on the p(2 x 2) alloy, but the sticking probability is reduced, compared to Pt(111), by an increase in the steric constraint on the dissociation site. Sticking on the (root 3 X root 3)R30 degrees alloy is inefficient at thermal energies with a threshold of similar to 280 meV, below which the sticking probability falls exponentially. The increase in the barrier to D-2 dissociation occurs as the stable, high coordination Pt-3-D binding sites are lost by formation of the (root 3 X root 3)R30 degrees alloy. Despite the large activation barrier, sticking is dominated by the vibrational ground state with the barrier occurring in the entrance channel, before the D-2 bond has stretched. Departures from a normal energy scaling indicate that the dissociation site is localized in the unit cell and we suggest favorable dissociation sites on the alloy surfaces. Estimates for the heats of adsorption, obtained by comparing activation energies to adsorption and desorption, indicate an abrupt decrease in the D binding energy as the Pt-3 sites are lost. We show that sticking and desorption parameters are consistent with an increasing steric constraint for adsorption/desorption on the alloy surfaces as the Sn content is increased and an increase in the barrier to dissociation as the stable Pt-3 sites are lost by alloying.