The effects of surfactants on bentonites have been of great scientific interest for many years. Even though quaternary alkylammonium salts (QAS) have been studied, very few data are available on the comparative performance of different chain-length QAS for the modification of the surface properties and adsorption properties of bentonites. The objective of this study was to investigate the effect of chain length on the adsorption of cationic surfactants onto bentonite. The surface and adsorption properties of different chain-length QAS, i.e. hexadecyltrimethylammonium bromide (HTAB, C16), tetradecyltrimethylammonium bromide (TTAB, C14), and dodecyltrimethylammonium bromide (DTAB, C12), to produce organo-bentonites (OB) were studied. The concentrations of QAS were selected based on the cation exchange capacity (CEC) of the clay mineral. Zeta potential, swelling, and viscosity measurements and scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) analyses were used to explain the changes in surface properties. The results indicated that the best modification of bentonite was obtained using a 16-carbon chain length QAS (HTAB) in a 1:1 ratio of QAS to CEC. The basal spacing at this concentration was measured to be 22.19 Å, which also corresponded to the maximum adsorption density. The OB produced at this concentration showed the best hydrophobic character based on the swelling tests in toluene. The extent of hydrophobicity and adsorption density was correlated with the CEC and alkyl chain of the QAS. All these properties were used to elucidate the mechanism of modification governing the bentonite/QAS system.