Illite is known to show highly selective Cs adsorption and can control the migration of Cs in a natural environment. However, the local environment and dynamic behavior of Cs with other ions on illite are not well known. To investigate this system, both NMR and theoretical calculation were used for the study of Cs with other alkali cations on the surface of illite. The order of selectivity for Cs adsorption on illite against other alkali cations is generally Li > Na > K > Rb. The fraction of Cs assigned to inner-sphere complexes as estimated by NMR is in the order of Na > K > Rb > Li, which is different from the order of Cs selectivity, and the difference becomes greater with decreasing Cs molar fraction in solution. This order of inner-sphere complex fraction of Cs is in good agreement with that found by theoretical calculation. The result of MD simulation shows the highest molarity peak for all competing alkali cations at 2-4 angstrom regardless of their fractions and species, but with different distribution and distance from the illite surface. Our results demonstrate that the Cs complexes on the surface of illite are highly dependent on the other hydrated competing alkali cations.