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 twotypes of organobentonites. Bentonites modified with short-chain alkyl functional groups(e. g., TMA) sorb organiccontaminants primarily by an adsorption process, in which the sorbed amount decreases with increasing sterichindrance of the organic compound and the process exhibits a competitive effect because of steric hindrance in multi-solute 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 organiccompounds may induce a cosorptive effect. In the latter case, the measured distribution coefficients of organiccompounds between organobentonites and water( log Kd ) are positively correlated with the octanol-water partitioncoefficients of the compounds (logKow) and the sorption process exhibits no competitive effect in multi-solutesystems.