Thin films of nickel-carbon prepared by d.c. magnetron sputter deposition have previously been shown to exhibit passivity against corrosion as well as electrocatalytic activity towards the hydrogen oxidation reaction of the low-temperature acidic fuel cell. Pure nickel dissolves rapidly under such conditions. Xray photoelectron spectroscopy (XPS) of Ni-C films containing between 13 and 64 at.% Ni, and polarized at +0.15 V(SHE) in 1.5 M H-2 SO4 at room temperature demonstrates that this passivity is not due to oxide formation, but rather to the presence of the carbon component. XPS identifies the Ni component as being in the metallic state, both before and after polarization in acid. The carbon component comprises a range of phases, with a graphitic or graphenic component being responsible for passivation.(C) 2014 Elsevier Ltd. All rights reserved.