Four commercially annealed tubing heats of 2/sup 1///sub 4/ Cr-1 Mo steel were aged at 454, 510, and 566/sup 0/C for various times up to 20,000 hr. Tensile tests were made at room temperature and at the aging temperature on specimens from each heat for each aging condition. Creep tests were made on specimens from two of the heats. Thermal aging significantly changed both the room temperature and the aging temperature tensile properties, the effect being greatest for tests at the aging temperature. The amount and magnitude of the strength changes varied from one heat to another. After aging for 20,000 hr at 454 and 510/sup 0/C, the strength of one heat increased. However, the other three heats showed decreases in yield strength and ultimate tensile strength of up to 40 percent, the maximum effect occurring at 566/sup 0/C. Similar large changes in creep-rupture properties were also observed.