Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems (soils, sediments, aquifers, rivers, lakes, and many remediation operations). This chapter provides a survey of the environmental and substrate characteristics that govern redox transformations in aquatic systems, and it suggests methods for estimating the thermodynamic and kinetic properties for redox reactions involving organic contaminants. The scope of this chapter is limited to non-photochemical, abiotic processes; photochemical processes are the focus of chapters 14 and 15, and microbial transformations are discussed in Chapter 12. Chapters focusing on estimation of properties for redox reactions involving inorganic substances have been published previously (1).