Solubility in water is an important property of surfactants for many purposes, including surface tension determination and (eco)toxicological studies. Surfactants have been categorized as “difficult to test”; the challenges associated with the determination of solubility in water will be discussed. OECD TG 105 is based on the definition of solubility in water as “saturation mass concentration”. For most surfactants, there is no saturation concentration, but rather formation of micelles or other surfactant aggregates. Unfortunately, there is a misconception that surfactant micelles are equivalent to droplets of hydrophobic material. This is not true, as there is a rapid exchange of surfactant molecules between those in a micelle and those present as single molecules in the bulk water – a true thermodynamic equilibrium. In addition, surfactants are able to solubilize hydrophobic substances within their micelles, enhancing their bioavailability.