Synthetic pyrethroids are strongly hydrophobic compounds, and their toxicity in sediment is regulated by phase distribution among the sediment, dissolved organic matter, and water phases. In the present study, we spiked and equilibrated four pyrethroids in two sediments, and we characterized their phase distribution as a function of contact time. The freely dissolved concentration measured by solid-phase microextraction was only a small fraction (<16.3%) of the total pore-water concentration as determined by liquid-liquid extraction. The fraction of the freely dissolved concentration was significantly greater in the freshwater sediment (1.7-16.3%) than in the marine sediment (1.1-4.2%) following 9 d of equilibration, and it decreased substantially with contact time to less than 5% at 30 d after sediment dosing. Consequently, the apparent organic carbon partition coefficient (Koc) and dissolved organic carbon partition coefficient (Kdoc) values increased significantly over the contact time, especially in the freshwater sediment, suggesting that phase distribution was not at equilibrium after 9 d of equilibration. If only the freely dissolved concentration is bioavailable, these observations suggest that contact time after sediment dosing may greatly affect the bioavailability and, hence, the toxicity of pyrethroids. Therefore, a long contact time (> or = 30 d) is recommended for sediment toxicity testing of this class of compounds. The dependence of bioavailability on contact time also implies that test conditions must be standardized to allow comparison between laboratory-dosed samples and field samples.