Precipitate development in two heats of HT-9 steels (84425 and 91354) during stressed and stress-free thermal aging treatments was studied. Stress-free thermal aging was conducted from 923 to 1033°K for 500 to 10 000 h while stress rupture conditions of 10.4 to 337 MPa stress at 811 to 1033°K for 60 to 17 754.8 h were utilized. Microdiffraction and energy dispersive X-ray spectroscopy techniques were used to analyze the precipitates. The carbide, M23C6, was the predominate precipitate phase in all conditions, with the exception of stressed specimens tested at 1033°K, where most of the M²³C⁶ was replaced by chromium nitride (Cr²N). Cr²N was found only in stressed specimens at and above 977°K. The presence of Laves phase below 977°K produced a compositional variation in chromium in M23C6 and in the metal carbide phases of the MX type. Sigma phase was found only after long time testing at 811°K at a high stress level. The MX phases generally were found in both heats under all conditions. These phases were vanadium-rich in the stress rupture specimens, while titanium-rich in some of the stress-free thermally aged specimens.