X-ray reflectivity, cyclic voltammetry, and scanning tunneling microscopy (STM) are used to examine the structure of alpha -SiW12O44- or silicotungstic acid (STA) adsorbed on Ag(100) in acid solution. The voltammetry shows that STA passivates the Ag surface relative to electron transfer to a solution redox species. STM images reveal the formation of a series of lattice structures, one of which can be associated with a commensurate (root 13 x root 13)R33.69 degrees structural model. X-ray reflectivity measurements show uniquely that STA orients with its four-fold axis perpendicular to the Ag(100) surface and that the center of the STA molecule is 4.90 Angstrom above the top laver of the Ag substrate. Analysis of bond lengths leads to a footprint of STA on Ag(100), in which the four terminal O atoms are located near the hollow sites and have a Ag-O bond length of 2.06 Angstrom. This bond length is consistent with a strong covalent interaction between STA and the Ag surface.