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toxic properties of the individual compounds within the group. For this RA, gasoline components were selected fir detailed evaluation on the basis of their biological properties and environmental mobilities. Alkanes are characterized by single bonds between- carbon atoms. They*may be arranged in straight chains or branched chains . The major alkanes in gasoline are n-butane, n-hexane, isobutane, isopentane, and 2-methylpentane (SWRCB 1987) . In general, these compounds have only moderate water solubility. The smaller straight--chained alkanes have a high tendency to volatilize but as the number of carbons increases, t1:e vapor pressure and water solubility decrease and there is a greater tendency to bind to soil. Branched alkanes tend to have greater water solubilities than the corresponding straight-chain alkanes . In general, the alkanes have low human toxicity (Sandmeyer 1981a; NAS 1982; ACGIH 1984) . There is no evidence that alkanes are mutagenic, teratogenic, or carcinogenic (Sandmeyer 1981a) . Although alkanes constitute a major fraction of gasolines, they are not considered in further detail in this RA because of their low toxicities and relatively low potential to migrate from a spill site. Alkenes are characterized by double or triple bonding between carbon atoms. on a weight basis, the alkenes make up a smaller percentage of gascline than the alkanes. The major alkenes in gasoline are trans-2-pentene and 2-methyl-2--butene (SWRCB 1987) They have moderate water solubilities and moderate-to-high vapor pressures (based on their low boiling points) . Alkenes have little inherent human toxicity (Sandmeyer 1981a; NAS 1982) . Because of the generally low concentrations, their limited mobilities and very low toxicities, alkenes will not be further evaluated. J C1cloalkanes are characterized by single bonds between carbon l c,� •- atoms, with the carbon atoms arranged in rings . Cycyoalkanes 14