The reactivity of nominally clean polycrystalline Ni toward gas-phase and Liquid anhydrous hydrofluoric acid (AHF) has been examined by X-ray photoelectron spectroscopy (XPS). To avoid contamination with atmospheric components, experiments were carried out using a portable ultrahigh Vacuum (UHV)-compatible chamber to transfer Ni specimens between a UHV system that houses the XPS spectrometer and an additional UHV-compatible chamber where the actual exposures were performed. For exposures of ca. 10 min, the extent of surface oxidation of clean Ni to form NiF2, as calculated from Ni 2p and F is XPS spectra, was found to be significantly higher for gs, compared to liquid-phase AHF. Thin (ca. 100 nm), mirror-like Ni films sputtered onto the surface of a sapphire disk yielded upon exposure to gas-phase AHF (after contact with the laboratory atmosphere) scanning electron microscope images displaying a coherent, patchy structure with raised junctures. This behavior was ascribed to internal compressive stresses induced by the formation of a nickel(II) fluoride layer on the metal surface.