A comparative study of chemical etch rates in diluted HF or a mixture of HF, H2O, and HNO3 (P etch) was performed on conventional thermal silicon oxides (1050–1120 °C; O2 pressure ≊1.1 atm; one type with addition of 0.02% C2H3Cl3) and buried oxide layers. The latter were formed by single or multiple implanting n‐ and p‐type (100) Si maintained at ≊600 °C with 150–200‐keV O+ ions to a dose of ≊1.8×1018 cm-2, and subsequent annealing at 1250–1325 °C. The multi‐implanted samples had three implants of a dose of 0.5–0.8×1018 O+ ions cm-2 each, separated by anneals but no final anneal. One of the thermal oxides was additionally irradiated with As+ ions to a dose of 1014 cm-2. Accurate mechanical thickness measurements revealed variations in etch rate between the different oxides, which provide direct evidence for a structural and/or stoichiometrical difference between thermal and buried oxides. The data indicate that the buried oxide is in a densified state as compared to thermal oxide. Also O+ and As+ irradiation‐induced bond strain was clearly observed in buried oxide and thermally grown oxide, respectively. Additionally, plots of the etch rate as a function of oxide thickness reveal detailed information on structural variations of the oxide layers as a function of thickness. This is in contrast with the generally assumed homogeneity of most of these layers.