Concrete durability is affected by the pH values of the cement pore solution. The pH influences transport properties, rebar corrosion rate and the possible occurrence of alkali silica reaction. Thus, knowledge on the interaction of alkali ions with C-S-H is decisive. We investigated experimentally the influence of KOH and NaOH on C-S-H properties. The presence of KOH and NaOH have a similar effect on both the aqueous and the solid phase: i) increase of the pH value; ii) lower Ca concentrations in solution and precipitation of additional portlandite; iii) higher Si concentrations in solution and dissolution of amorphous silica; iv) shorter silica chains in C-(N,K-)S-H; v) increase of the basal peak intensity in XRD, which indicates an in-creased stacking in c direction. The amount of bound alkalis increases with increasing alkali hydroxide concentrations and is higher at low Ca/Si in the C-S-H.