Kinetics in water of some chemical and photochemical reactions postulated as key transformations in the environmental mercury cycle were investigated. Decomposition of dimethylmercury (DMM) and diphenylmercury (DPM) by acids and mercuric salts was investigated. Theoretical evidence indicates that loss of elemental mercury or DMM at the air-water interface can be important in turbulent systems. Dimethylmercury, methylmercuric chloride, methylmercuric hydroxide, and methylmercuric ion were not decomposed by sunlight, but phenyl-mercury and sulfur-bonded methylmercuric species were readily decomposed to inorganic mercury. Detailed equilibrium calculations indicate that the sulfur-bonded methylmercuric species are the predominant species in natural waters. Quantum yields for these reactions are presented along with a technique for calculating sunlight photolysis rates from laboratory data. The chemical literature concerning the kinetics of chemical and photochemical decomposition of organomercurials is reviewed. (Modified author abstract)