Transverse-field muon spin rotation (TF-μSR) spectra have been obtained for muoniated free radicals formed by positive muon irradiation of a series of four-membered cyclic silylenes. Muon-electron hyperfine coupling constants (Aμ) were determined from the spectra; values range from 28.5 to 59.3 MHz. The radicals are formed by muonium (Mu) addition to the silylenes, in a fashion similar to the expected reaction of the H atom. However, it is unclear a priori which is the site of addition. Accordingly, density functional theory (DFT) calculations were carried out for two classes of radicals: silyl radicals formed by H addition to the backbone C atom of the four-membered ring and alkyl radicals formed by H addition to the unsaturated Si(II) center. By comparison of the predicted proton hyperfine constants with the experimentally determined Aμ values it is possible to assign structures to each detected radical. Examples of both addition sites were found. © 2015 American Chemical Society.