Abstract We present the results of a study of the modification in electron capture rates on metal surfaces with an electronegative impurity: oxygen. Electron capture is investigated on the example of O − and F − formation in oxygen and fluorine ion/atom scattering. O − and F − ion fraction measurements were made for Mg and Al surfaces exposed to O 2. Changes in electron capture rates were continuously followed from the limit corresponding to a metal with submonolayer chemisorbed oxygen to those on an oxide film. A comparative measurement was made for an MgO(100) surface. It is shown that in the low coverage, chemisorption range local effects due to specifics of the electronic structure at the adsorbate site can strongly attenuate electron capture. This effect, akin to surface poisoning, may be attributed to changes in the local density of states and modification in the positions and widths of the anion level near the adsorbate. At high exposures corresponding to oxide formation electron capture is very efficient and can be understood in terms of a gas-phase-like, localized non-resonant charge exchange mechanism between the incident atom and, for example, an MgO lattice O anion. At intermediate coverages, the behaviour of the ion fractions is affected by appearance of oxide islands.