Nonionic Organic Compounds (NOCs) sorb to swelling clay minerals by multiple mechanisms that produce almost a continuum of sorption maxima when NOCs with different functionalities are compared. NOCs with multiple functional groups that are each capable of complexing aqueous interlayer cations sorb very strongly (up to 10% by weight for di- or tri-nitrobenzenes) to K- and Cs-saturated smectites. Substitution of progressively less polar substituents for one of the nitro-groups results in a gradual decrease of the maximum sorption to K-smectites. Presence of a single polar substituent, such as a carbonyl group, similarly results in moderate sorption. In all these cases, the polar moieties on the NOC are seen by FTIR spectroscopy to interact with interlayer cations. This study quantifies and begins to infer mechanisms for sorption of less polar compounds by K- and Cs-smectites. Reference saponite and beidellite smectites were saturated with Ca2+, Na+, K+, or Cs+. Batch isotherms were measured for sorption of three triazine herbicides (atrazine, simazine, and metribuzin) and trichloroethene (TCE) to the homoionic clays. The saponite clay sorbed a larger fraction of each triazine pesticide from aqueous solution than did beidellite clay. The smaller layer charge in saponite (vs. beidellite) presumably resulted in a less crowded interlayer with more siloxane surface available for adsorption. Interlayer cations strongly affected atrazine adsorption: Cs-saturated clays sorbed more triazines than clays saturated by K+, Na+, or Ca2+. Similarly, Cs-saturated saponite sorbed the most TCE, while Ca-saturated smectite sorbed the least. We hypothesize that the stronger sorption of triazines and TCE by the Cs-smectite can be attributed to the lower hydration energy and hence smaller hydrated radius of Cs+, which expands the lateral clay surface domains available for sorption. The maximum sorption of atrazine to Cs-smectites was greater than 1% by weight for the Cs-saponite. On most K-smectites, we observed atrazine sorption of between 0.5 to 1.0 g/kg clay, although maxima were not reached. We showed that sorption of atrazine to K-beidellite was directly related to interlayer spacing: The K-beidellite that was air-dried and rehydrated sorbed ten times as much atrazine as a K-beidellite that was never dried, and the only detectable difference between the two clays was that the air-dried system exhibited d001-spacings of 12.5 and 15.5 Å while the never-dried system showed d001-spacings of 15.5 and 18.5 Å. Thus, the 12.5-Å spacing apparently contributes most of the atrazine sorption in the K-smectite system. This interpretation is consistent with our hypothesis that Cs-smectites are stronger sorbents for NOCs like atrazine because they are most likely to have 12.5 Å d001-spacings. This hypothesis implies that there is a significant hydrophobic component for sorption of weakly polar NOCs to smectites, and this hydrophobic mechanism is most effective when the NOC is most completely removed from water. The 12.5 Å d001-spacing accomplishes maximal dehydration of aromatic NOCs because both molecular planes of the NOC must interact only with smectite siloxane surfaces and not with water. In accord with this hydrophobic mechanism, atrazine (with one more methylene C) consistently sorbed slightly more to the smectites than did simazine. There may also be a complexation component to the adsorption mechanism, because molecular dynamics simulations showed that negatively charged ring-N and -Cl atoms of atrazine interacted with multiple interlayer ions in both inner- and outer-sphere complexes, while most of the atrazine molecule occupied siloxane surfaces between charge sites in the smectite layers.