Sodium mass transfer deposits up to 10 mils in thickness have been observed on Type 304 stainless-steel, cold-leg surfaces after 10 000 h of steady-state operation. The magnitude of deposition has been shown to be time and location dependent. Deposit thicknesses will continue to grow over the 40-yr design life of the intermediate heat exchanger (IHX), with the only mechanisms which may tend to limit deposit thicknesses being flow shear forces at the wall, and stresses induced by thermal transients. Some of the physical and chemical properties of typical mass transfer deposits collected from primary and secondary sodium systems have been characterized. The composition was found to be highly dependent on the location in the system, and the first materials to precipitate were rich in chromium, whereas deposits located further downstream contained large amounts of manganese. A 50°F drop in temperature was normally sufficient to initiate precipitation of these deposits. Interpreted in terms of the reactor system, this would indicate that the step change in temperature, encountered when bypass streams are mixed with core coolant sodium, may be large enough to initiate deposition in the isothermal hot-leg piping. In addition to the metallic constituents, carbon concentrations as high as 2% have been measured in the deposits together with significant quantities of nitrogen and oxygen. Since carbides, oxides, and nitrides typically exhibit lower thermal conductivities than metallic elements or alloys, it was expected that the deposits would represent a significant heat transfer resistance. Experiments were designed to measure the degradation in the heat transfer coefficient due to corrosion product deposition in small sodium loop systems. Application of these results to IHX and steam generator designs indicated a 9% reduction in heat transfer.