The composition of thin SiO2films was measuredin situby MeV He+backscattering spectrometry after sequential sputtering with 2‐keV Ar ions. Results indicate that the relative concentration of the constituents in the film changes during sputtering. However, differential analysis showed that after an initial period the ratio of the removed constituents agrees with the original composition. We interpret the change in composition with an existing model according to which only the composition of a thin surface layer changes due to the dissimilar sputtering ratios of the two constituents, while the composition in the remainder of the specimen is unchanged.