CoNi alloy nanoparticles, formed by co-evaporation onto freshly cleaved highly oriented pyrolytic graphite (HOPG) surfaces, have been studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS). X-ray photoelectron spectroscopy (XPS), and scanning electron (SEM) and atomic force (AFM) microscopies. ToF-SIMS detected CoxNiy fragments, indicating alloy formation. Even under ultra-high vacuum, the nanoparticles reacted with residual C- and O-containing gases to form surface contaminants (carbides, oxides, etc.) as revealed by both XPS and ToF-SIMS. On prolonged exposure to air, both the zero-valent metal and carbide peaks of each component decreased with time, as each metal reacted with atmospheric oxygen; as with the pure metals, the Co component of the alloy was the more reactive. (C) 2010 Elsevier Inc. All rights reserved.