An experimentally practical and precise flocculation-based method was developed, tested, and applied to determine phenanthrene and 1,2,4-trichlorobenzene sorption with NIST SRM 2975 diesel particulate matter. Following an initial equilibration period, polyaluminum chloride (PACI) solution was added to the sorption tubes in order to facilitate the formation of flocculated aggregates of soot particles. After separation of the solids through centrifugation, supernatant concentrations were determined as with conventional batch methods. The flocculation-based method was tested on three kinds of soot and then used to evaluate sorption kinetics and equilibrium with SRM 2975. Kinetic results showed that wetting of the soot required more than 20 days, but that 60 days was sufficient to achieve equilibration with both water and phenanthrene. Sixty-day isotherms for both phenanthrene and 1,2,4-trichlorobenzene were strongly nonlinear. At approximate 10(-3) of solubility, carbon-normalized distribution coefficients (Koc) were 10-20 times higher than those for absorption to sediment organic matter. Measurements at closer to solubility indicated much lower Koc, suggesting a total sorption capacity at aqueous solubility that is of similar magnitude to that in sediment organic matter. Independent analysis of extractable hydrocarbons suggests that absorption into a native hydrocarbon phase was not a major component of sorption.