High resolution Si-29, Na-23, and Cs-133 solid-state NMR spectroscopy was used to study the interaction of sodium and cesium chloride salts with calcium silicate hydrates (C-S-H), the main components of hydrated Portland cement. Spectra were collected at different levels of moisture and suggested that Cs+ and Na+ in contact with C-S-H behaved differently, mainly because of their different hydration energy, which is smaller for Cs+ than for Na+. An interpretation of the results is given, using a conceptual model based on an analogy between C-S-H and smectites, which present analogous structural features. The results suggest that cesium and sodium have an affinity for the C-S-H surface. In hydrated C-S-H, the cations may be located in a diffuse ion swarm. In dry C-S-H, sodium cations would form outer-sphere complexes, i.e., they would be adsorbed with their hydration sphere, whereas cesium cations would be adsorbed as inner-sphere complexes with two distinct environments, one without chloride and the other with chloride anions in their coordination sphere. At low Ca/Si ratios, the mechanism involved for cationic adsorption is probably a compensation of the negative surface charge of C-S-H.