In this paper, we use the distribution of the artificial radionuclide, 137Cs, to investigate mixing and ventilation in the Black Sea. Timeseries data of vertical radionuclide distributions are combined with model simulations in order to follow the magnitude and depth of penetration of surface oxic water into intermediate depths, below the oxic/anoxic interface. Simulated data are produced by a three-dimensional circulation model that includes a new parameterization of the Bosphorus inflow/plume to simulate the effects of Mediterranean water on internal mixing. A radioactive tracer model for pre-Chernobyl 137Cs (weapons testing 137Cs) and Chernobyl 137Cs is coupled to the circulation model. The model results are compared to field data collected between 1986 and 1992. The main output from the simulations is the identification of the contribution of entrainment in the mixing of surface waters into subsurface layers. The trend of tracer penetration into the deeper layers following isopycnal surfaces is well demonstrated in the model and is consistent with the known circulation and physics of the Black Sea. The correlation between the activities of radionuclides and salinity, found in the field data, is supported by the simulations. These model results illustrate that the time-space abundance of the existing field data is sufficient for reconstructing the distribution of tracers in space and time, provided reliable estimates of Black Sea circulation exist. ¿ 1999 American Geophysical Union