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ORGANOTINS Organotins are compounds with at least one tin-carbon bond. In most organotin compounds, tin is in the tetravalent oxidation state. Four series of organotin compounds are known: R Sn, R SnX, R SnX , and RSnX wherein R is usually a butyl, octyl, or phenyl g?oup, and X i? co& my chloN de, fluoride, oxide, hydroxide, carboxylate, or thiolate (CEC 1978). The possible molecular composition and structure of the R groups are virtually unlimited (Laughlin et al. 1985). At least 260 organotin compounds presently are known, of which 36 are listed as toxic chemicals (Watanabe 1980). Except for some methyltin compounds, all organotins are manufactured (Laughlin et al. 1985). Most commercially used organotins are characterized by low mobility in the environment because of low aqueous solubility, low vapor pressure, and high affinity for soils and organic sediments (Blunden and Chapman 1986). Solubility data for organotin compounds are incomplete. In general, their solubility in water is limited to about 5 to 50 mg11, but they are very soluble in many common organic solvents (WHO 1980). The presence of chloride in seawater reduces the solubility of tributyltin and triphenyltin compounds, probably by association with the hydrated cation to form the covalent organotin chloride (Blunden et al. 1985). Organotin compounds are analyzed in aqueous media by spectrophotometric, fluorometric, and electrochemical techniques. However, if picomole per liter concentrations are required, additional techniques must be used. More work needs to be done on analytical detection methods of organotins in sediments and biota (Thompson et al. 1985). Methylation of inorganic and methyltin compounds has been reported with the formation of mono-, di-, tri-, and tetramethyltin compounds. In addition, tributylmethyltin and dibutylmethyltin species have been found in harbor sediments, which suggests that some butyltin compounds may be methylated in aquatic systems (Guard et al. 1981; Thompson et al. 1985; Donard et al. 1987). Abiotic and biological degradation of organotins generally occurs through sequential dealkylation or dearylation (Zuckerman et al. 1978; WHO 1980; Smith 1981b; Chau et al. 1984; Blunden et al. 1985). Organotin compounds undergo successive cleavage of4tin-carbon bonds to v timately produce inorganic tin as folkyws: R4Sn > R3SnX > R2SnX2 > RSnX3 > SnX The reaction rate, k, usually proceeds as k4 >> k3 >> k2 = kl. The breakigg of a Sn-C bond can occur by a number of different processes, including ultraviolet irradiation (UV), biological cleavage, chemical cleavage, gamma irradiation, and thermal cleavage (WHO 1980; Blunden and Chapman 1982, 1986; Blunden et al. 1985; Thompson et al. 1985). In general, UV and biological cleavage are the most important processes. The main abiotic factors that seem to limit organotin persistence in the environment are elevated temperatures, increased intensity of sunlight, and aerobic conditions (Table 1). A probable environmental degradation scheme for tributyltin 5