The n-octanol-water partition coefficient (commonly referred to as either KOW or POW) is a widely used property for assessing the partitioning behaviour of chemicals in the environment. Over the last several decades, a number of single-parameter relationships based on KOW have been developed to estimate the fate, behaviour and effects of chemicals in the environment, such as sorption to soils and sediment, bioavailability, bioconcentration, and ecotoxicity. Octanol is regarded as a model solvent that mimics lipid tissues in organisms and humans, and organic carbon in soils and sediments. Octanol has, therefore, been widely used as an appropriate surrogate for the partitioning of chemicals from aqueous media to organic matrices (Leo et al, 1971; Hansch and Leo, 1979). The generation of KOW is integral to the ERA of chemical substances and therefore the reproducibility and accuracy of this value is of paramount importance. In this section, the standard methods for the measurement of KOW are evaluated in terms of their applicability to the ERA of ionisable compounds and the most appropriate methods are recommended. A review of the key regulatory triggers associated with KOW, together with an examination of the role of ionisation on bioconcentration potential is provided further below.