Highlights • Oxygen adsorption driving Ni of Rh(110)/Ni into subsurface sites. • Reduction with H 2 restores Ni content of surface. • Formation of macroscopic 2D-Ni oxide islands at surface defects. • Evidence for large nucleation barrier for 3D-Ni oxide on Rh(110). Abstract The behavior of ultrathin layers of nickel (0.5 ≤ θ Ni ≤ 1.4 monolayers (ML)) on Rh(110) has been studied in oxidation/reduction experiments with O 2 and H 2 at T = 770 K. The reaction has been followed with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning photoelectron microscopy (SPEM). The adsorption of oxygen leads to a large decrease (up to 90%) of the Ni signal in AES. Subsequent titration with hydrogen restores nearly the initial Ni signal. The peculiar behavior of Ni on Rh(110) upon O 2 adsorption is attributed to a large nucleation barrier for Ni oxide formation on Rh(110). This barrier leads to the preferential formation of chemisorbed oxygen on Rh sites forcing Ni into subsurface sites. SPEM reveals that also small NiO particles with a diameter of ≈1 µm develop during O 2 adsorption. For comparison, a Rh(110) surface with a thick Ni layer of an estimated thickness of about 8–9 ML has been prepared. On this thick Ni layer Ni oxide develops spontaneously during O 2 adsorption in the in 10−6 mbar range at T = 770 K. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]