Abstract The chemical etch rates in diluted HF or a mixture of HF, H2O and HNO3 (P etch) of conventional thermal silicon oxides (1050–1120°C) and buried oxide layers were compared. The latter were formed by single or multiple implanting n- and p-type (100) silicon maintained at approximately 600°C with 150–200 keV O+ ions to a dose of approximately 1.8 × 1018 cm-2, and subsequent annealing at 1250–1325°C. Accurate mechanical thickness measurements revealed variations in etch rate between the different oxides, providing direct evidence for a structural and/or stoichiometric difference between thermal and buried oxides. The data indicate that the buried oxide is in a densified state compared with the thermal oxide. Additional, plots of the etch rate vs. oxide thickness reveal structural variations of the oxide layers as a function of thickness, which is in contrast with the generally assumed homogeneity of SiO2 layers.