The atomic geometry of the Al(100)-c(2x2)-Li phase formed by adsorption of 1/2-ML Li on Al(100) at room temperature has been determined by analysis of extensive low energy electron-diffraction (LEED) measurements. The structure is found to be a binary surface alloy, in which Li atoms occupy fourfold-coordinated substitutional sites formed by displacing every second Al atom in the first layer of the substrate. The surface structure is very similar to the (100! plane of the metastable, bulk Al3Li alloy. An analysis of experimental LEED data for clean Al(100) leads to the conclusions that the first interlayer spacing is expanded by about 2% with respect to the bulk value, and that the vibrations of Al atoms in the first layer are about twice as large as for atoms in the bulk. Adsorption of Li leads to a contraction of nearly 6% of the first Al-Al interlayer spacing, and further enhances the vibrations of Al atoms in the first, mixed Al/Li layer [S0163-1829(99)16331-8].