Using a 20 mm bore, grease-lubricated, ball bearing at 200°C and 1450 rev./min, we find that bearing metal distress occurs during operation in pure hydrogen or helium whereas no distress is observed during similar tests with oxidizing atmospheres such as air or unpurified carbon dioxide. In hydrogen, the surface damage is mainly in the form of extensive pitting and flaking of the balls of the bearing, each pit or cavity having a “cod's eye” appearance indicative of hydrogen embrittlement. With helium, on the other hand, the damage is in the form of fine surface pitting which metallurgical examination leads us to believe occurs by a mechanism different from that operative in hydrogen. This preliminary work shows that further investigation is essential, to study the phenomena over a considerably wider range of conditions.