Local variations in the reactivity of NO on the Ru(0001)-(1 x 1)H surface have been probed with a supersonic molecular beam. The presence of adsorbed hydrogen atoms creates an elastic scattering channel in the specular direction, concomitant with a suppression of the initial sticking probability. Argon is scattered in a qualitatively similar, but instrumentally limited, specular peak from the same surface. The well characterized position of the hydrogen atoms allows the scattering to be related to collisions at the threefold hollow sites within the surface unit cell.