A high-level theoretical study on the thermochemistry of hydrochlorofluorosilanes has been carried out using the Gaussian-3 (G3), and its computationally less expensive variant G3(MP2), methods. In this paper, we report the heats of formation of 15 silanes, their adiabatic ionization energies (IEs), electron affinities (EAs), and proton affinities (PAs), as well as the acidities of 10 hydrosilanes. Good to excellent agreement with the available experimental data is found for essentially all calculated quantities. The only exceptions are those involving the SiF4+ cation, such as the IE and PA of SF4. The origin of this failure is not immediately clear, even though this failure is not confined to the G3 methods. Because many of the thermochemical data for the industrially important silanes are not available in the literature, the results reported here may be taken as reliable estimates.