The reduction of Ru oxide was examined as a way of cleaning the cap layer of multilayer mirrors in extreme ultraviolet lithography (EUVL). Ru deposited on a Si surface was oxidized using electron cyclotron resonance (ECR) O2 plasma, and then treated with atomic hydrogen generated with a hot tungsten wire. An analysis of the surface composition by x-ray photoelectron spectroscopy and Auger electron spectroscopy revealed that atomic hydrogen removed the Ru oxide resulting from the ECR O2 plasma treatment. Additionally, atomic force microscopy observations showed that this treatment caused no increase in the surface roughness of the Ru. This indicates that the surface oxidation of EUVL mirrors is reversible, and can largely be eliminated by using atomic hydrogen and the proper cap layer.