Energetics and structural aspects of the reaction between F- and silacyclopropane (2), silacyclobutane (3), silacyclopentane (4), and silaspiropentane (5) have been studied by theoretical calculations. All species included have been fully geometry optimized at the RHF level using a 6-31++G** basis and analytically calculated gradients. Energies have been calculated at the MP4(SDTQ) level using the same basis. Predicted fluoride affinities have been corrected for differences in (unscaled) zero-point energies calculated analytically using the same basis. In order to obtain information on the sensitivity of predicted fluoride affinities to changes in basis sets, calculations including full optimizations on the silacyclopropane system with the basis sets 6-31G**, MC-311G**, and MC-311++G** have also been carried out. The predicted equilibrium geometries are practically independent of the basis sets used, but predicted fluoride affinities require diffuse functions. The affinities predicted are roughly (within 1.5 kcal/mol) the same for doubly and triply split valence shells. Correction for BSSE on the silacyclopropane system using the 6-31++G** basis amounts to 1 kcal/mol. The fluoride affinities obtained at the highest calculational level (without BSSE corrections) are 43.2, 43.2, 37.3, and 45.5 kcal/mol for 2, 3, 4, and 5, respectively. The fluoride affinity of the cyclic molecule c-H2Si-CH2-NH (6), isoelectronic to 2, has been estimated to be 5 3.2 kcal/mol at the same calculational level, and previous calculations give a corresponding value of 61.8 kcal/mol for the cyclic molecule c-H2Si-CH2-O. The fluoride affinity of the most stable form of the noncyclic molecule H2Si(CH3)2 (7) is predicted to be 30.0 kcal/mol. Our combined geometry and energy results indicate that the strength of the Si-F bond formed, rather than release of ring strain in the adduct, is the important contributor to the fluoride affinity.