The enhancement of secondary ion (SI) yields (Cu+, Ni+) by CO and O-2 adsorption on Cu/Ni(1 0 0) surfaces has been studied as a function of Cu coverage (0-1 monolayers (ML)) using secondary ion mass spectrometry (SIMS). The experiments measure an enhancement factor (EF), which is the ratio between the SI yield (for Cu+ or Ni+) measured after CO (or O-2) exposure, to the corresponding SI yield measured for the clean surface prior to the exposure. Typical EFs for both Cu+ and Ni+ are found to be similar to100 for CO/Cu/Ni(1 0 0) and similar to15 for O/Cu/Ni(1 0 0). A simple tunnelling model, with parameters tested previously for clean Cu/Ni(1 0 0) surfaces, was used to estimate the influence of work function variations on the magnitudes of EFs produced by CO and O-2 adsorption. The predictions of this model are smaller (by an order of magnitude) than the observed EFs, which suggests that the EFs are too large to be attributed to work function variations alone. EFs produced by CO adsorption (10 L exposure) depend exponentially on the CO uptake by the Cu/Ni(1 0 0) surfaces. However, Ni atoms, to which CO bonds exclusively, show a greater propensity than Cu atoms to form SIs. The observation of an EF of similar to100 for Cu+ emitted from 0.1 ML Cu/Ni(1 0 0) + CO, and of similar to5 for Ni+ emitted from 1 ML Cu/Ni(1 0 0) + O-2, implies the operation of non-local SI enhancement mechanisms, since in neither case does the SI precursor atom bond directly to the adsorbate species. However, for both CO and O-2 adsorption, the highest EFs seem to be associated with structural situations which favour direct bonding between metal and adsorbate atoms. (C) 2002 Elsevier Science B.V. All rights reserved.