Because of the large surface area and common occurrence of smectites in the clay fractions, they are important in sorption/desorption reactions of organic pollutants entering soils and sediments. Results of sorption and desorption of atrazine by Al- and Ca-saturated smectite reported here showed that Al-saturated smectite sorbed much higher amounts of atrazine than Ca-saturated smectite. Al-saturated smectite sorbed 3820 mg kg (super -1) as compared to 1902 mg kg (super -1) by the Ca-saturated smectite during 5 consecutive sorptions from 10 ppm atrazine solution. Sorption isotherms over 2-10 ppm atrazine concentration range were nonlinear in all cases. Freundich coefficients, K f , obtained from the sorption isotherms were much higher for Al- than Ca-saturated smectite; K f varied from 405 to 3035 for Al-saturated and 100 to 306 for Ca-saturated. The pH values of Al- and Ca-saturated smectites were 3-4 units higher than the pK a (1.68) of atrazine, which suggests that atrazine was sorbed as neutral molecules. Stronger H-bonding between the more polarized H 2 O associated with the trivalent Al ion than the divalent Ca ion is likely responsible for the greater sorption by Al-smectite.