The thermal densification of austenitic stainless steel and the effect of minor alloy variations on this phenomenon have been investigated. Increasing the carbon content of AISI Type-316 stainless steel can produce density changes >0.1% after aging at 1500°F for 100 h. Large additions of phosphorus (0.04 wt%), boron (0.006 wt%), and nitrogen (0.13 wt%) produce no significant density change upon aging. It was concluded that the thermal densification phenomenon observed in austenitic stainless steels is due to the formation of carbides and is linear with the carbon content of the alloy.