Organobentonites synthesized by replacing the metal ions in bentonite with cetyltrimethylammonium (CTMA) or tetramethylammonium cation (TMA) were investigated for their behaviors to sorb benzene, toluene, p-xylene from water. The results served to distinguish the sorption mechanisms (adsorption or partition) of the two types of organobentonites. Bentonites modified with short-chain alkyl functional groups (e.g., TMA) sorb organic contaminants primarily by an adsorption process, in which the sorbed amount decreases with increasing steric hindrance of the organic compound and the process exhibits a competitive effect because of steric hindrance in multisolute systems. In contrast, the sorption of contaminants to organobentonites modified with long-chain alkyl groups (e.g., CTMA) occurs by partition process without exhibiting a competitive effect, and the additional organic compounds may induce a cosorptive effect. In the latter case, the measured distribution coefficients of organic compounds between organobentonites and water (log Kd) are positively correlated with the octanol-water partition coefficients of the compounds (logKow) and the sorption process exhibits no competitive effect in multi-solute systems.