In high resolution depth profiling of thin layers by Secondary Ion Mass Spectrometry (SIMS), there is a strong influence of preferential sputtering on depth resolution which is frequently overlooked. However, for preponderant atomic mixing, an extended version of the so called Mixing-Roughness-Information depth (MRI) model reveals the correlation between preferential sputtering and atomic mixing. While the latter is constant for a given ion-solid interaction, the effective mixing length scales inversely with the relative sputtering rate on the depth scale and is mainly responsible for the observed changes in profile widths. As an experimental example, recently published results by Moiseeev a al. [1] are re-evaluated and quantified by the MRI model, thus confirming the model prediction for the case of preferential sputtering of GaAs with respect to Mn.