Hydrogen, as a green energy carrier, seems promising for green energy transition initiatives. For this reason, hydrogen-based industries are growing, and machines like compressors and pumps are widely used for hydrogen-handling purposes. Rolling element bearings, typically made of 100Cr6 bearing steel, are vital components for the operation of rotary pumps and compressors. However, hydrogen is known to reduce the lifespan of 100Cr6 bearings by causing embrittlement. Hence, it is crucial to investigate the impact of hydrogen on 100Cr6 bearings operating in a hydrogen-rich environment.Previous literature has focused on hydrogen-promoted subsurface damage in 100Cr6, with less attention to early-stage surface-initiated damage. Since initial surface-initiated damage will eventually grow to a larger damage feature and ultimate failure of the bearings, it is important to understand how hydrogen exposure and concentration affect the early-stage surface-initiated damage of 100Cr6. Therefore, this study aims to investigate the effect of hydrogen concentration in 100Cr6 bearing steel to identify critical hydrogen levels at which surface-initiated damage occurs due to hydrogen embrittlement.For this purpose, the surface of the test specimens (100Cr6 rollers) was analyzed using a 3D optical profilometer and then electrochemically pre-charged with different charging current densities, followed by rolling/sliding tribo-testing using a micro-pitting rig (MPR). The lubricant used for the micro-pitting rig tests were PAO68 and PAO2.Post-test analysis was performed using 3D optical profilometry, light optical microscopy, and scanning electron microscopy to assess the effect of hydrogen concentration on surface damage and to detect possible subsurface deformation. Surface damage was quantified in terms of area coverage using MountainsMap. Additionally, thermal desorption mass spectroscopy (TDMS)analysis was conducted to measure hydrogen content in the pre-charged roller samples.The results showed that tribo-testing of un-charged samples led to low surface damage, while surface damage increased with higher charging current densities. Higher surface damage was observed at 7.1 𝑚𝐴/𝑐𝑚2  (40 𝑚𝐴) and 10.6 𝑚𝐴/𝑐𝑚2 (60 𝑚𝐴) charging current densities,which seem to be pre-charging current density values related to critical hydrogen amounts.Furthermore, no subsurface defects were observed for the tribo-test duration of 5 hours with a 10-minute initial stabilization period. The friction coefficient and wear mechanisms wererepeatable for all tests. Thermal desorption spectroscopy analysis results revealed that 5 −6 𝑝𝑝𝑚 hydrogen is the critical amount, resulting in significant surface-initiated damage to the roller samples.