The changes that occurred in composition, microstructure, and mechanical properties of a sample of welded AISI Type 304 stainless-steel pipe that had been exposed to liquid sodium for 21 000 h within the range 300 to 800°C have been determined. The results of the investigation showed that under the liquid-sodium service conditions studied, Type 304 steel pipe did not undergo any marked deleterious change in mechanical properties. The tensile strength of the exposed sample was above the minimum value specified by the ASTM for welded Type 304 steel pipe, whereas the yield strength was at the minimum value specified. The results of standard flattening tests on specimens from the exposed sample indicated that they could be flattened without cracking. However, a compositional change occurred on the inner surface of the pipe (the surface that had been exposed to the liquid sodium for 21 000 h). Nitrogen, sulfur, and sodium were absorbed, and a relatively brittle layer, ∼0.05 mm thick, was formed at the inner surface. Thus, when Type 304 steel is used for liquid-sodium service, sodium should be kept as free as possible of contaminants such as nitrogen and sulfur, in addition to oxygen.