Quantitative Secondary Ion Mass Spectrometry (SIMS) determination of oxygen incorporation in epitaxial AlGaAs/GaAs layers is strongly affected by the presence of oxygen adsorbed on the surface. By flooding the sample with oxygen isotope O18 during the analysis, one can account and correct for the influence of the adsorbed oxygen O16 species. After the background subtraction, the oxygen content can be determined from the measured ratio of AlO to Al or O to Al signals, knowing the relative sensitivity factors between these quantities as obtained from measurement of an O16 implanted standard.