<p>High-purity hydrogen is essential in fuel cell vehicles (FCVs), which require strict control of impurities, including residual moisture content. Therefore, it is imperative to thoroughly investigate the influence of trace moisture on the friction and wear behaviors of tribomaterials. In this study, the friction and wear behaviors of polytetrafluoroethylene (PTFE) composites containing 20% of polyphenylene sulfide (PPS) in a high-purity hydrogen gas environment were investigated under various contact pressures, sliding speeds, and trace-moisture contents at the parts per million (ppm) level. A pin-on-disk tribometer installed in a high-vacuum chamber was used for the tests, and the disk material was SUS440C stainless steel. The results revealed that the trace-moisture content significantly affected the wear rates of the PTFE/PPS composites. At a moisture content of ~ 20 ppm, the wear rate was reduced to one-tenth the wear rate at a low-moisture content of ~ 1 ppm. This reduction is attributed to the effective formation of a PTFE-based transfer film supported by interfacial tribochemical reactions, accompanied by the formation of metal fluorides. Conversely, at a moisture content of 1 ppm, insufficient stabilization of the PTFE transfer film leads to the adhesion of PPS to the counterface, resulting in increased wear. Despite the variations in wear rates, the friction coefficient remained relatively invariant, regardless of the moisture level. These findings highlight that even trace amounts of moisture, often neglected owing to the assumption of hydrogen purity, can critically affect tribological performance. Without understanding this moisture effect, severe wear may occur unexpectedly, which poses a risk for FCV-related hydrogen systems.</p> Graphical Abstract <p></p>