Radionuclide concentration data taken during decontamination of the primary reactor coolant system at Three Mile Island by a feed-and-bleed process have provided information on future defueling operations. Analysis of the radiocesium concentrations in samples taken at the letdown point indicates general circulation within the primary system, including the reactor vessel and both steam generators. A standard dilution model with parameters consistent with engineering estimates (volume, flow rate, etc.) accurately predicts the radiocesium decontamination rates. Unlike cesium, the behavior of other principal soluble radionuclides (90Sr and 3H) cannot be readily described by dilution theory. A significant appearance rate is observed for 90Sr suggesting a chemical solubility mechanism. The use of processed water containing high 3H for makeup causes uncertainty in the interpretation of the 3H analysis.