Cationic surfactants are used in a wide range of applications such as fabric softeners, cosmetics, hair care products, and disinfectants. Because removal by wastewater treatment is incomplete, 1 cationic surfactants have been widely detected in the aquatic environment. Toxicity to aquatic organisms is relatively high; studies have shown that submicromolar concentrations (i.e., < ~100 μg/L) in water can cause toxicity to a number of aquatic organisms. Research on their environmental fate and distribution is therefore highly needed. Sorption is a particularly important process for cationic surfactants because the surface of natural materials such as soils and sediments is often negatively charged. Past studies have indicated that cation exchange is the primary mechanism of soil sorption of cationic surfactants. Natural organic matter and clay minerals are the main contributors to soil cation exchange capacity (CEC) and are thus expected to play the major role in the sorption of organic cations. There are many studies in the literature on the sorption of cationic surfactants by clay minerals, reporting on various aspects such as the influence of surfactant and clay mineral types, coexisting solutes, temperature, and pH on sorption, as reviewed by Ishiguro and Koopal. While previous studies investigated sorption over a wide range of surfactant concentrations, the range was usually from μM to mM, with emphasis on a high region, around the adsorbent CEC or even up to the surfactant critical micelle concentration (CMC). This may be because clay sorption exhibits characteristic shifts passing through the CEC and CMC, which have been a focus of research. The use of analytical techniques with low sensitivity may also be a factor limiting the concentration range. However, to understand the environmental behavior of these contaminants, knowledge of the sorption behavior at sub-μM concentrations is required, as this is the range typically found in aqueous environments and reported as the toxicity threshold.