Monolayers of AlAs, in a matrix of GaAs grown by molecular beam epitaxy, were characterized using 1.0 keV O2+secondary ion mass spectrometry (SIMS) employing sample rotation to reduce uneven sputtering and improve depth resolution. Under optimal conditions, a full width at half maximum resolution of 2.6 nm was obtained. This resolution is discussed in terms of surface roughening, the cascade mixing model, and preferential sputtering. Cascade mixing predicts well the mixing estimated from experimental measurements. In addition, using this SIMS characterization procedure, mixing from 280 keV Ar+bombardment was studied as a function of depth. The mixing with depth varied as dictated by cascade mixing. However, quantitative estimates of the mixing were only ∼0.2 of the observed values.