The adsorption of hexadecyltrimethylammonium (HDTMA) in both swelling (montmorillonite) and nonswelling (kaolinite) layer silicates was studied by adsorption isotherms, X-ray diffraction and electrophoretic mobility. The objectives were to determine the effects of clay type and solution composition on the adsorption of cationic surfactants and to reveal the relation between the structure of the adsorbed surfactant layer and the stability of surfactant-clay complexes. We found that HDTMA adsorption by montmorillonites was more complex than by a nonswelling clay such as kaolinite. The major complexity arose from the fact that the structure of adsorbed HDTMA layer in the interlayers of montmorillonites depended strongly on the initial degree of day dispersion. Highly dispersed clays (e.g., Na-montmorillonite in dilute NaCl solutions) lead to a random HDTMA distribution on the surfaces, larger distances between silicate layers, and hence reduced lateral interactions between the alkyl chains of adsorbed HDTMA. Accordingly, cation selectivity coefficients were low at low HDTMA loadings and increased dramatically as HDTMA loading increased, resulting in a unique S-shaped adsorption isotherm, which has not been observed for nonswelling clays. Initially flocculated clays (e.g., Na-montmorillonite in concentrated NaCl solutions or Ca- or Cs-montmorillonite) lead to HDTMA segregation and more compact adsorption layers, which enhanced the lateral interactions between alkyl groups. As a result, the cation selectivity coefficients were high even at low HDTMA loadings and changed little as loading increased, similar to that for nonswelling clays. Both cation exchange and hydrophobic bonding were involved in HDTMA adsorption at high loadings (e.g., >0.75 CEC). Charge density of clays, ionic strength of the bulk solution, and the type of companion anions all affect the adsorption of HDTMA via hydrophobic bonding.