The forces of electrical imaging strongly polarize the surface of colloidal silica. I used X-ray scattering to study the adsorbed 2-nm-thick compact layer of alkali ions at the surface of concentrated solutions of 5-nm, 7-nm, and 22-nm particles, stabilized by either NaOH or a mixture of NaOH and CsOH, with the total bulk concentration of alkali ions ranging from 0.1 to 0.7 mol/L. The observed structure of the compact layer is almost independent of the size of the particles and the concentration of alkali base in the sol; it can be described by a two-layer model, that is, a ∼6-8-Å-thick layer of directly adsorbed hydrated alkali ions with a surface concentration of ∼3 × 10 18 m -2, and a ∼13-Å-thick layer with a surface concentration of sodium ions of ∼8 × 10 18 m -2. In cesium-enriched sols, Cs + ions preferentially adsorb in the first layer replacing Na + ; their density in the second layer does not depend on the presence of cesium in the sol. The difference in the adsorption of Cs + and Na + ions can be explained by the ion-size-dependent term in the electrostatic Gibbs energy equation derived earlier by others. I also discuss the surface charge density and the value of surface tension at both the air/sol and the hexane/sol interfaces.