Five sediment cores were taken in 1990-1991 from the sedimentation basins of Lake Ontario and analyzed for the radionuclide 210Pb and hydrophobic organic compounds (HOCs) in order to determine the accumulation, inventory, and diagenesis of these HOCs in the sediments. Two of these sites were sampled earlier in 1981, allowing the study of diagenetic processes affecting these HOCs over a decadelong interval. The shape and details of HOC sediment profiles agreed with the HOC production and usage history, despite evidence of bioturbation in the cores. The 210Pb chronology showed a mixed depth of 2-5 cm, but mixing by deposit-feeding oligochaetes and benthic organisms was insufficient to homogenize the sediment over the time scale of HOC inputs. Recent HOC accumulation rates and inventories showed significant variability among cores, which was removed when corrected for 210Pb-based sediment focusing. This suggests that particle-reactive compounds like HOCs are mixed and distributed evenly throughout the lake basins and that site-specific differences are due to differing amounts of sediment delivered via focusing ofsedimentto depositional basins. Comparison of 1981 and 1990 sediment cores showed expected downcore movement of HOC profiles due to 9 yr of accumulated sediment mass with effectively no loss or gain in mass.