Initial thermal oxidation of silicon and thermal desorption of silicon oxide have been investigated by Si 2p core-level photoemission with synchrotron radiation. The surface reaction processes are discussed from the difference in the chemically-shifted components, especially suboxides. On the thermal oxidation, time evolutions of suboxide intensities show a distinct temperature dependence, which is explained by two growth modes for the oxidation: first-order Langmuir-type adsorption mode and two-dimensional island growth mode at oxidation temperatures below and above 650 °C, respectively. On the thermal desorption, the spectrum for the thermal oxide followed by annealing at 1000 °C for 30 s in vacuum, whose thickness is nominally estimated at 3.7 Å, are compared with the 3.3 Å-thickness thermal oxide without the process of the annealing. The intensities of the suboxide components are considerably different between them, which can be explained by the formation and lateral growth of the voids in the annealed oxide layer.