A laboratory heat of an improved boiler steel containing 0.13 percent C, 1.36 percent Mn, 0.27 percent Mo, 0.03 percent Cb, and 0.010 percent N was prepared; creep-rupture properties, resistance to temper embrittlement and resistance to hydrogen attack were investigated. The rupture strength was much higher than that of carbon steel and columbium-treated carbon steel, but was somewhat lower than that of two European carbon-0.3 percent Mo boiler steels. Creep-rupture ductility was high. The experimental steel exhibited high toughness, especially in the normalized and stress-relieved condition. No temper embrittlement was induced by step-cooling normalized or normalized and stress-relieved material. Good resistance to hydrogen attack was revealed by tests in a hydrogen autoclave at a pressure of 1000 psi (6.9 N/mm2); the steel retained the original Charpy impact toughness after exposures up to 5000 hr at 900 deg F (480 deg C) and 500 hr at 1000 deg F (540 deg C). No blistering or fissuring were observed.