Abstract: Polymer treatment of bentonite contributes to enhance effectively the chemical compatibility of vertical barriers. Reported researches are limited by the impacts of organic contaminant and kind of polymer on the anti-seepage performance of polymer treated bentonites. The swell potential and permeability of sodium carboxymethyl cellulose(CMC)treated bentonites subjected to phenol solutions are evaluated via swell index and modified filtration tests. Fourier transform infrared spectrum analysis is conducted to understand the reasons for the improvement of the chemical compatibility due to characteristic groups on CMC polymer chains. The swell index(SI) of CMC treated bentonite is 1.5 to 2.5 times higher than that of untreated bentonite(CB)for a given phenol concentration. The hydraulic conductivity(k) of CB is reduced by an order of magnitude after CMC treatment. The bentonite treated by CMCⅢ shows the best anti-seepage performance. Molecular weight and degree of substitution of CMC have no significant effect on both SI and k in this study. The main reasons for such improvementare as follows:(1)Polymer hydrogels in CMC treated bentonite is able to block macro-pores between bentonite particles. (2)Characteristic groups(carboxyl group and carboxylic acid group) can adsorb phenol through esterification reaction and hydrogen bondings existed between phenol hydroxyl group and water molecules in hydrogels hinder the migration of phenol in solution,leading to reduce the detrimental effect of phenol on the double layer thickness of montmorillonite.