In oxidizing environments, the toxic and radioactive element uranium (U) is most soluble and mobile in the hexavalent oxidation state. Biotite was reacted with aqueous solutions of U(VI) in the presence of 1–100 mM NaCl or CaCl₂ at pH of 3.0, 5.0, and 7.0. U adsorption was slightly greater in CaCl₂ solutions relative to NaCl solutions. The reason is that the interlayer K⁺ ions were exchanged to a larger degree by Ca²⁺ than by Na⁺ ions. XRD and transmission electron microscope (TEM) investigations revealed the presence of interlayer U(VI) species, suggesting a slow but considerable diffusion of U(VI) even into deep interlayer regions of the biotite particles. U(VI) may be stabilized in the interlayer regions in a long-term process. This can imply that once U is diffused into the interlayer region of biotite, U migration will be significantly retarded in subsurface environments.