Natural waters near Earth’s surface commonly exist far from redox equilibrium and hence hold a thermodynamic drive for the oxidation of some aqueous species at the expense of others, which are reduced. The rates at which such oxidation and reduction reactions occur in the natural environment are described by kinetic laws, which may account for heterogeneous catalysis or promotion by enzymes. This chapter shows how to incorporate redox kinetics into multicomponent chemical reaction models and gives a fully worked example of how such models can be applied.