In multi-phase matrices, phase distribution determines the bioavailability and hence the ecotoxicological effects of a contaminant. For a hydrophobic pollutant, it is well recognized that its availability for bioaccumulation or toxicity is reduced due to association with colloidal particles and dissolved organic matter (DOM). The reduction in bioavailability is influenced by both the quantity and quality of the colloidal and DOM phases. As the fine colloids and DOM are essentially inseparable from the aqueous phase, quantifying bioavailable concentrations is a great challenge. This presentation will discuss the use of solid-phase microextraction (SPME) for detecting bioavailable concentrations of pyrethroids in surface aquatic systems. The bioavailability of several pyrethroid compounds was measured with polydimethylsiloxane (PDMS)-coated fibers in surface waters with dissolved organic matter or suspended solids, and in sediment porewater. Chemical accumulation in the PDMS fiber was further correlated with bioaccumulation or toxic effects observed in aquatic invertebrates. The studies show convincingly that PDMS can be used as a “biomimetic” surrogate to estimate the bioavailability of pyrethroids in runoff effluents, surface water, and sediments.