The effects of the presence of a cationic surfactant on the sorption of two typical nitrogen heterocyclic compounds (NHCs) on swelling and nonswelling phyllosilicates were investigated. The addition of a cationic surfactant to a NHC-phyllosilicate system can either solubilize the sorbed NHC molecules or immobilize the suspended NHC molecules, depending upon the concentration and sequence of the surfactant added. The presence of surfactant molecules on the clay mineral surfaces promoted sorption of more neutral NHC molecules due to hydrophobic effect, and this resulted in a considerable increase in sorption at high pH conditions. Both surfactant and NHC molecules were sorbed not only on the external surfaces of the phyllosilicates but also on the interlayer spaces of swelling-type clay minerals, and the resulting change in basal spacings indicates the competition between the two groups of molecules in the interlayer space. Copyright 2000 Academic Press.