The most common experimental approach in aquatic toxicity testing is to spike the exposure medium directly with the pure substance for sufficiently water-soluble organic compounds, or with a biocompatible and water-soluble solvent as an intermediate for poorly water-soluble organic compounds. However, defining and controlling their bioavailable, freely dissolved concentrations (Cfree) is a challenge for strongly sorptive compounds, including cationic surfactants. Cfree, which is usually not measured in toxicity tests, may be lower than the nominal concentration (Cnom) because of sorption of the chemical to dissolved organic matter, glass surfaces, and test organisms or degradation during the testing time, resulting in low test accuracy. Therefore, such toxicity data of the chemicals might not be reliable and useful for environmental risk assessments, and there is an urgent need for new approaches to overcome these challenges in existing test protocols.