We discuss new opportunities to understand processes at the solid–liquid interface using resonant anomalous X-ray reflectivity (RAXR). This approach is illustrated by determination of element-specific density profiles at mica surfaces in aqueous electrolyte solutions containing Rb+ and Sr2+. The total interfacial electron density profile is determined by specular reflectivity (i.e., reflected intensity vs. momentum transfer, q, at an energy, E, far from any characteristic absorption edge). RAXR spectra (i.e., intensity vs. E at fixed q) reveal element-specific ion distributions. Key differences in the interaction of Rb+ and Sr2+ with mica are observed using resonant anomalous X-ray reflectivity: Rb+ adsorbs in a partially hydrated state, but Sr2+ adsorbs in both fully and partially hydrated states.