The formation of the sigma phase (an intermetallic compound generally designated as FeCr) in high-chromium ferritic irons after certain thermal treatments is an established fact. This is indicated by the classical investigation of Bain and Griffiths (1) and by other investigators in this field (2, 3, 4). However, the precipitation of this phase directly from a balanced austenitic stainless steel matrix has not yet been completely accepted. Primarily, this paper attempts to add to the increasing evidence (5, 6, 7) that sigma may form directly from highly alloyed austenite, and it has the further purpose of determining the effects of such precipitation on the mechanical properties of the alloy. Numerous stainless steel tubing sections which have had prolonged exposure at elevated temperatures in various industrial services have been examined by the Research Department of The Babcock & Wilcox Tube Co. Under the proper conditions of temperature and time, a rapidly etching constituent tentatively identified as sigma phase has been detected in types 302B, 309, 310, 316, 317, 321, and 347. It should be pointed our that, in spite of many examinations of exposed metal, sigma phase has not been found by us in wrought type 304 material. Of the previously mentioned stainless steel grades, types 317 and 309 retain ferrite at room temperature while the remainder, with the exception of type 310, all contain, in appreciable amounts, elements with strong ferrite-forming tendencies (silicon, molybdenum, titanium, and columbium). In addition to their ferrite-forming tendencies, titanium and columbium exert a further accelerating influence on sigma formation by stabilizing the carbon, thereby permitting maintenance of a higher effective chromium content in the matrix. To avoid, as much as possible, the influence of ferrite on sigma phase formation, a steel which has no appreciable amount of ferrite-forming elements (chromium excluded) and contains enough nickel to render it austenitic under all heat treatments was chosen for the set of experiments reported herein. Because it meets the foregoing prerequisites and is, in addition, a highly utilitarian alloy finding application in many important industries, type 310 (25 per cent Cr - 20 per cent Ni) alloy was used for this investigation.