The sorption of selenite (Se(IV)) at trace (sub-ppm) to high concentrations on periclase (MgO) under high-pH conditions (pH > 10) was examined by macroscopic sorption experiments and nanoscale solid phase analyses via transmission electron microscopy and X-ray absorption spectroscopy. The maximum distribution coefficient (K<sub>d</sub>) of Se(IV) on MgO was 100 L/g, the highest among any reported mineral sorbents at pH > 10. Since MgO is a metastable phase under ambient conditions, it transforms instantaneously to brucite (Mg(OH)<sub>2</sub>) in solution. Se(IV) was preferentially and homogeneously distributed onto the newly formed Mg(OH)<sub>2</sub>. The Mg(OH)<sub>2 </sub>formed thin flake-like platelets, which appeared to be aggregates of nanoscale Mg(OH)<sub>2</sub> particles, the primary alteration product of MgO. The chemical form of Se(VI) adsorbed on nanoscale particles was outer-sphere complexes. Therefore, the outer-spherically adsorbed Se(IV) was occluded into the large flake-like Mg(OH)<sub>2</sub> particles, resulting in its effective isolation from the solution.