The thermal stability of acetaldehyde on Si(111)7×7 was studied with HREELS, XPS, UPS, and TPD techniques. Acetaldehyde molecules were found to adsorb molecularly on the surface at 120 K, yielding HREELS peaks at 98, 124, 144, 185, and 380 and UPS peaks at 5.7, 7.6, 10.0, and 13.9 eV below Ef. Analysis of the XPS O1s (shoulder at 531; 532 eV) and C1s (288 and 285 eV) spectra indicated that the acetaldehyde was partially dissociated at 120 K. At ∼350 K, the initial HREELS peaks at 185 and 144 meV diminished while peaks at 100 and 208 meV emerged, which indicated the desorption and/or dissociation of the C - C bond and the formation of Si - O and Si - CH3 species. The UPS peaks at 5.7 and 10.0 eV diminished, and a new peak at 6.5 eV dominated the spectra from 350 to 1150 K. The parent mass was found by TPD to desorb at 320 K. At 500 K, the 100 meV HREELS peak vanished and a new peak at 270 meV became noticeable indicating the breaking of the C - H bond and the formation of Si - H (D) species. TPD results indicated that H (D) species desorbed at around 800 K. By 1150 K, peaks at ∼110 meV (HREELS) and ∼7 eV (UPS) were the only significant features not found in the clean Si(111) spectra. In the XPS spectra, initial multiple features had converged by 350 K, and with increased temperature both the resulting O1s and C1s peaks shifted toward lower binding energy. By 1150 K, the O1s peak had disappeared, leaving silicon carbide on the substrate.