Sediment profiles of ¹³⁷Cs in 12 lakes and of radionuclides (⁶⁰Co, ¹³⁴Cs, ²²⁶Ra) added experimentally to four lakes at the Experimental Lakes Area, northwestern Ontario, were inconsistent with sediment chronologies derived from ²¹⁰Pb distributions. Peaks in the ³⁷Cs profiles were sometimes shallower than the depth corresponding to the 1963 fallout maximum (based on ²¹⁰Pb chronology) and sometimes ¹³⁷Cs peaks were not observed at all. Expected peaks in the profiles of experimentally added nuclides were similarly absent in three of four lakes. The absence of these peaks at depths corresponding to their time of maximum input to the lakes probably resulted from the initial uptake of the nuclides by littoral sediments followed by the winnowing over periods of several years of nuclide-tagged fine particles to the deeper parts of the basins where the cores were collected. Sediment focussing is manifested by ²¹⁰Pb sediment inventories at the coring sites greatly exceeding inventories that can be supported by atmospheric deposition of ²¹⁰Pb in this area.¹³⁷Cs and the experimentally added nuclides were found much deeper in the cores than the depths corresponding to the time of their initial inputs to the lakes based on ²¹⁰Pb chronologies. Diffusion of the nuclides in the pore waters could explain some of their downward penetration. However, in some cases, the profiles could only be fit if the effective diffusion coefficient increased with depth. This situation might reflect mixing of these high porosity sediments during core collection or extrusion.Distributions of ¹³⁷Cs and of the experimentally added nuclides do not provide valid chronologies which can be used to date other features recorded in the sediments of these lakes. Although unsupported ²¹⁰Pb profiles in these lakes provided excellent fits to classical steady-state sediment accumulation models, ²¹⁰Pb profiles too may have been affected by sediment mixing or core disturbance. Further studies are necessary to test the reliability of ²¹⁰Pb-based sediment chronologies in high porosity sediments such as those studied at ELA.