Heterogeneous reactions are important contributors to the transformation of organic chemicals in the environment and are increasingly being used in the treatment of polluted water. Kinetic models are presented here that describe selected environmentally relevant thermal and photochemical reactions in particle/water systems, including reactions in approximately equilibrated heterogeneous systems and in systems involving reactive site saturation. The effects of slow intersite diffusion on heterogeneous reactions are discussed as are recent models that describe the kinetics of such processes. To illustrate the application of the kinetic models, examples are provided of kinetic treatments of thermal and photochemical reactions involving organic chemicals in aqueous suspensions of sediments, metal oxides, or algae. Thermal reactions that are discussed include the hydrolysis of sorbed organic chemicals and the reduction of organic compounds in anaerobic sediments.