Passive dosing is an alternative approach based on the equilibrium partitioning concept introduced in toxicity testing and could solve the challenges mentioned above. The method typically uses a polymer sorbent phase that is biocompatible, does not react with the test chemical, and acts as a chemical partitioning source to the exposure medium. The passive dosing method is expected to have several advantages when applied in ecotoxicity testing. For instance, Cfree can be defined and controlled based on the partitioning equilibrium between the passive dosing phase and exposure medium (i.e., with a partition coefficient between the sorbent and water phases, Ksorbent/water). Moreover, there is no oversaturation/precipitation issue, and spiking solvent can be avoided. Cfree can remain constant throughout the test if the sorbent material has a large enough sorption capacity for the test chemical (i.e., high mass or volume and high Ksorbent/water), even if some chemical loss occurs, e.g., due to degradation. Cfree is measured directly in the medium sample or, if not possible, can be estimated from Csorbent and known Ksorbent/water, improving the reliability of toxicity data. Passive dosing methods have been successfully applied in toxicity testing for nonpolar hydrophobic organic compounds such as polycyclic aromatic hydrocarbons. However, sorption mechanisms of ionic compounds are different from those of nonpolar hydrophobic compounds and a passive dosing method for ionic compounds has not been established yet. Although passive sampling methods have recently been applied in ecotoxicity tests to obtain Cfree forBACs, 8, 40 , to the best of our knowledge, there is no prior research on a passive dosing method for cationic chemicals, including cationic surfactants with a long alkyl chain (i.e., #C ≥14), for which a tool to control the aqueous exposure concentration is urgently required.