This paper reports on the site-specific decomposition behaviour of N2O onthe clean Si(100)2 × 1 and Si(111)7 × 7 surfaces at room temperature, studied by spectroscopic differential reflectometry (SDR) and Auger electron spectroscopy (AES). It has been established that Si---O bond formation occurs at the toplayer Si atoms, mainly via attachment of the unsaturated broken bonds. In this way it is possible to determine the density of broken (dangling) bonds at the clean Si surface. Our results confirm the high dangling bond density at the 2 × 1 reconstructed Si(100) surface for which we assume a defect density of 15 ± 10%. The density of dangling bonds per 7 × 7 unit cell (49 atoms), 23 ± 4, is in agreement with the established 19 as revealed by current imaging tunneling spectroscopy (CITS).