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Recent studies related to the p‐type doping of GaInAs epilayers, grown by molecular‐beam epitaxy, mentioned erratic results concerning the electrical activity of beryllium (Be). This work reports on the secondary‐ion mass spectrometry analysis of moderately Be‐doped (between 5×1017 and 5×1018 cm-3 ) GaInAs/InP layers. Two of them were n type while they should have been p type. The comparison of Be depth profiles, obtained under either oxygen or cesium primary‐ion bombardment, and detailed study of the secondary‐ion mass spectra reveals that Be+ secondary‐ion useful yields are largely enhanced in n‐type samples when using cesium primary ions. Such behavior is attributed to the presence of varying oxygen contents in the samples, as confirmed by quantitative analyses. Comparison of Be and O concentrations indicates that the two elements probably form complexes inside the GaInAs matrix, providing a mechanism for the observed Be electrical inactivity. The use of positive secondary‐ion yield enhancement under cesium bombardment is generalized to other elements (magnesium) in other semiconductor materials, in particular to the case of Au‐Mn/GaAs ohmic contacts. Once again such ions reveal the presence of oxygen. Paradoxically, BeCs+ or MgCs+ molecular ions are not sensitive to the presence of oxygen. It is hypothesized that MCs+ species are more stable than MO+ . Such a differential effect is very helpful in characterizing the presence of oxygen in semiconductor materials.