The binding of Na+, K+, and Li+ by magnesium silicate hydrate (M-S-H) was investigated in batch sorption experiments. Sorption isotherms and cation exchange measurements indicated the binding of alkalis in cation exchange sites compensating the negative surface charge of M-S-H. Higher pH values led to further deprotonation of the silanol groups and a higher alkali uptake by M-S-H. No significant incorporation of alkalis in the main silica or magnesium oxide sheets was observed. However, the silica sheets were less polymerized in the presence of higher alkali hydroxide concentrations.