Singlet and triplet excited states of the title compounds were investigated experimentally, using electron-energy-loss spectroscopy (EELS), and theoretically, using density functional calculations that include configuration interaction (DFT/SCI). Both the tripler and the singlet spectra are well rationalized by the theory, permitting an assignment of the observed, broad bands and providing strong indication that DFT/SCI is suitable for the description of excited states in molecules containing third-row elements. The low-lying transitions in tetramethylsilane are found to be 4s and 4p Rydberg; the lowest valence states are higher, at 9.8 eV (triplet) and 10.5 eV (singlet). The lowest triplet band in hexamethyldisilane is found to be valence, the upper orbital having pi:symmetry. The lowest singlet state is 4s, albeit with zero oscillator strength. The lowest observed singlet band is 4p, Rydberg with substantial pi* valence admixture. The calculated density of excited states is high in tris(trimethylsilyl)silane, and Rydberg-valence mixing is prevalent. A high density of states is found also for tetramethoxysilane, the low-lying transitions being all Rydberg originating from the oxygen lone pair orbitals. Excitation functions for selected vibrational and electronically excited states are presented. The former provide the electron attachment energies. The latter indicate large cross sections for triplet excitation near threshold and, thus, imply substantial yield of triplet states under typical plasma conditions. The He I photoelectron spectrum of tetramethoxysilane is also presented.