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The elution of pyrene (Pyr) into an aqueous solution containing humic acids (HAs) was investigated using activated carbon (AC) as an adsorbent of Pyr. The apparent water solubility of Pyr that adsorbed to the AC was increased by the presence of HAs. To quantitatively interpret the water solubility enhancement of Pyr in the presence of HAs, the partition coefficient of Pyr was evaluated using a micellelike partition model that took into account the partition of Pyr between water and the HA phase. The relationship between the apparent water solubility of Pyr and the concentration of HA was not linear, as expected by that model. To explain this result, we introduced a Langmuir isotherm that assumed the re-adsorption of Pyr on the AC in a micelle-like partition model. The experimental data set fitted well to the derived equation, and the partition and adsorption coefficients were evaluated based on this model. The elution of trace amounts of Pyr from the adsorbent as affected by the presence of dissolved organic carbon was simulated using the derived coefficients.