Adsorption of Rb+ to the quartz(101)-aqueous interface at room temperature was studied with specular X-ray reflectivity, resonant anomalous X-ray reflectivity, and density functional theory. The interfacial water structures observed in deionized water and 10 mM RbCl solution at pH 9.8 were similar, having a first water layer at height of 1.7 +/- 0.1 angstrom above the quartz surface and a second layer at 4.8 +/- 0.1 angstrom and 3.9 +/- 0.8 angstrom for the water and RbCl solutions, respectively. The adsorbed Rb+ distribution is broad and consists of presumed inner-sphere (IS) and outer-sphere (OS) complexes at heights of 1.8 +/- 0.1 and 6.4 +/- 1.0 angstrom, respectively. Projector-augmented planewave density functional theory (DFT) calculations of potential configurations for neutral and negatively charged quartz(101) surfaces at pH 7 and 12, respectively, reveal a water structure in agreement with experimental results. These DFT calculations also show differences in adsorbed speciation of Rb+ between these