The present work reports the adsorption mechanisms of aniline and toluene in dry and hydrated montmorillonite (MMT-Na and MMT-Na-W) and tetra-methylammonium-cation-modified MMT (MMT-TMA) as determined through density functional theory. These theoretical investigations explicitly demonstrate that cation-pi interactions between Na+/TMA(+) cations and aromatics play the key role in adsorption of organics over MMT-Na and MMT-TMA. Weak hydrogen bonds between the H atoms of organics and basal O atoms of tetrahedral silicate also stabilize the location of organics. The combination of interactions between water and basal O atoms and between organics and water molecules in hydrated MMT complexes strengthens the adsorption of organics on MMT, resulting in higher formation energies in hydrated organically intercalated MMTs than in the corresponding dry complexes. The adsorption of organics also changes frontier orbital distributions and consequently promotes the preferential occurrence of reactions on the organics rather than on the MMT layers. These adsorption mechanisms predicted by theoretical investigation can be used to explicate the adsorption of aromatic organics on aluminosilicates with different external environment.