The effects of trace impurities in hydrogen environment were studied. A pin-on-disk apparatus set in a vacuum chamber equipped with a turbo molecular pump and a gas purifier enabled the control of trace water at 0.5-90 ppm in hydrogen environment. The significant influences were recognized on the coefficient of friction and specific wear rates of both ferrous materials and the influences varied by the types of ferrous materials. XPS analysis of sliding surfaces suggested the formation of iron oxide and hydroxide especially when the experiments were performed in higher water concentration. This is an abstract of a paper presented at the World Tribology Congress (Kyoto, Japan 9/6-11/2009).