Recent research reported that significant amounts of helium (about 8 appm) are entrained in extruded Type 304 stainless steel powder that has been rapidly solidified by centrifugal atomization with helium cooling gas. The entrained helium was reported to be directly related to a desirable improvement in properties of materials such as a notable resistance to grain growth, improved room temperature tensile properties with little reduction in ductility and improved creep properties.The purpose of this work was to study the effect of inert gases on the microstructure of powder-metallurgy (P/M) Type 304 stainless steel with different preconditioning heat treatments. The heat treatments performed on the specimens include 1 hour anneals at 900,1000,1100, and 1200C followed by water quenching. In addition, the 1100 and 1200C annealed specimens were subjected to aging treatments at 600C for 1000 hours. The TEM specimens were thinned by electropolishing at 13C using a solution of 5% perchloric acid in acetic acid.