Contaminant migration is strongly controlled by sorption reactions; thus, the behavior of anions, which are (almost) not sorbing under alkaline conditions, is an issue of environmental concern. This is especially relevant in the frame of low and intermediate-low radioactive waste repositories, where the pH generated by cement-based materials is hyperalkaline. Selenite (SeO3 2-) sorption on calcium silicate hydrate (C-S-H) phases-the main cement sorbing minerals-has been investigated by batch experiments, ζ-potential measurements, and thermodynamic modeling to elucidate retention mechanisms and possible competitive/synergetic effects of cation coadsorption. Selenite sorption was shown to be nonlinear and slightly increasing with the C-S-H Ca/Si ratio; precipitation of CaSeO3(s) was observed for Se concentration higher than 2 × 10-3 M. Indeed, the presence of Ca is essential to enable selenite retention under alkaline conditions. Progressive additions of Na2SeO3 or NaCl salt to the phases produced a change in the C-S-H surface properties, that is, a decrease in the ζ-potential, in apparent agreement with anion adsorption. However, this effect had to be also correlated to Na coadsorption, as Cl showed null retention on the C-S-H phases. At the same time, anion adsorption had a clear effect on the retention of other cations (Ba) in the system. The distribution coefficient of Ba (at trace concentrations) suffered a moderate decrease by the presence of Na+ and Cl-, but it was improved by the presence of Na+ and SeO3 2-, indicating complex competitive/synergetic effects between anions and cations. All of the experimental data were satisfactorily modeled considering a classical double-layer approach.