By combining in situ SIMS/RBS measurements, sputter yields variations and ionisation probability variations were measured in the transient region of a poly-Si0.8Ge0.2 layer bombarded with 12 keV O-2(+) at normal incidence. The goal of the experiment is to investigate if the prepeak in the Ge+ SIMS signal could be explained by sputter yield variations. The result of the experiment shows a minimum in the sputter yield variation of Ge during the build-up of the altered layer, which does not coincide with the minimum of the Ge+-signal. This suggests that the prepeak is not only a sputter yield effect but also due to a change in ionisation probability. (C) 2002 Elsevier Science B.V. All rights reserved.