Electronic and geometric properties of the adsorbate-substrate complex formed upon adsorption of methyl oxirane on Si(100)2×1 at room temperature are reported, obtained with synchrotron-radiation-induced valence and core level photoemission. A ring-opening reaction is demonstrated to occur, followed by a five-membered ring formation involving two of the Si surface atoms bound to a surface dimer. Core-level photoemission spectra support the ring-opening reaction and the Si-O and Si-C bond formation, while from the valence spectra a more extended molecular fragmentation can be ruled out. We discuss the most likely geometry of the five-membered ring.