The toxicity and accumulation of eleven moderately water-soluble metabolizable (MSM) or neutral lipophilic (NL) compounds were examined in the midge Chironomus riparius in tests without sediment (water only) and in treatments with sediment to which the test chemical was added directly to the water (spiked water) or directly to the sediment (spiked sediment). In general, the toxicity of NL compounds was significantly reduced when sediment was present regardless of whether the compound was added to the water or the sediment. The observed reduction in toxicity when sediment was present was attributable to sorption of the chemical to sediment, dissolved organic carbon (DOC), and particulates and was anticipated from literature K_ow and measured K_oc values. The toxicity and accumulation of MSM compounds in sediment and nonsediment systems were much more variable than for the NL compounds. For 1-naphthol, no significant difference in the distribution of parent chemical was seen in the water or sediment when used, although the toxicity of 1-naphthol was lowest when the compound was added directly to water in the presence of sediment. For the remaining MSM compounds, toxicity was significantly reduced only when the chemical was added directly to sediment. In general, partition coefficients were less useful in predicting the environmental behavior of MSM compounds. The most notable effect of sediment upon the MSM group was that the persistence of the parent compound was greatly increased in the sediments when the chemical was added to sediment.