A long-period X-ray standing wave (XSW) study of the reactivity of mineral surfaces in contact with metal(loid)-bearing aqueous solutions is presented. Experimental XSW fluorescence yield data for Pb(II) and Pb(II)/Se(VI) adsorption on alpha-Al2O3(0001) and -(1102) surfaces under in situ conditions are described, and the details of the data modeling approach are presented. XSW results for Pb(II) uptake (at [Pb] = 6-600 muM) on the (1102) and (0001) surfaces of alpha-Al2O3 in the absence of Se(VI) confirm that the (1102) surface has an overall higher affinity for Pb(II) uptake than the (0001) surface at pH 4.5. The presence of Se(VI) at 600 muM appears to result in a significant diffuse-layer accumulation of Pb, likely because of a large negative surface charge on the alpha-Al2O3(1102) surface, but to exert only a small influence at the (0001) surface. The observed reactivity differences are discussed in terms of the surface structures of the hydrated alpha-Al2O3(0001) and -(1102) substrates.