Our recent studies indicated that many agrochemicals demonstrated a great affinity with smectite clays. Exchangeable cations associated with clay minerals significantly influence pesticide sorption by controlling interlayer distance, the size of sorptive domains, and the ability of sorbate functional groups to interact with interlayer cations. Pesticide sorption is influenced not only by the intrinsic natures of smectite clays (e.g., surface charge density and location) and exchangeable cations but also by clay interlayer hydration status. The amount and type of salts present in aqueous phase is a determinant of clay interlayer swelling property and hydration status. In this study, we measured sorption isotherms of alachlor, atrazine, dichlobenil and diuron by K-SWy-2 in KCl solution and Ca-SWy-2 in CaCl2 solution at several levels of ionic strength. The results indicated that pesticide sorption by K-SWy-2 significantly increased with increasing aqueous KCl concentration. In contrast, sorption by Ca-SWy-2 from aqueous CaCl2 solution at different ionic strength remained nearly equal. The changes of interlayer hydration status emanated from aqueous ion type and strength will be evaluated for clues to explain the alterations of pesticide sorption by clay minerals.