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Sediment partition coefficients were measured for the commercial phthalate esters, disodecyl phthalate (DIDP) and ditridecyl phthalate (DTDP). The experimental procedure was based on the U.S. Environmental Protection Agency (EPA) Test Guideline 796.2750, “Sediment and Soil Adsorption Isotherm.” Three sediments were used: EPA 8 (0.15% organic carbon), EPA 18 (0.66% organic carbon), and EPA 21 (1.88% organic carbon). The Freundlich equation was used to calculate organic carbon-normalized sediment/water partition coefficients (Koc), which averaged 2.86 E5 and 1.20 E6 for DIDP and DTDP, respectively. By interpolation, the Koc of DINP is estimated as approximately 6E5. In general, these Koc values did not correlate well to either sediment or chemical properties. This lack of correlation suggested that the measured Koc values are suppressed, potentially as a function of experimental conditions. On the basis of these data, it was decided to investigate the dependence of Koc on sediment solids concentration and dissolved organic carbon. Analysis of these and earlier reported partition coefficient data indicated that measured Koc values for phthalate esters obtained in shake-flask experiments exhibited an inverse dependence on solids concentration. These results were consistent with partitioning models that are discussed. Depending on compound hydrophobicity, the particle-corrected Koc values were from one to three orders of magnitude higher than the measured Koc values. Therefore, if partition coefficient values obtained by using Test Guideline 796.2750 or similar shake-flask procedures are not corrected for solids effect, the estimates of the sediment pore-water concentration of the chemical is likely to be overestimated. DINP vapor pressure is very low which suggests very limited volatilization from the terrestrial compartment.