The radicals BunCl3-nSn. were generated by photolysis of the appropriate [cyclopentadienyltin compounds, Bun(C5H5)SnCl3-n, and their reactivity towards alkenes and alkyl bromides was monitored by e.s.r. spectroscopy. Towards both reagents, the reactivity decreases as the number of chloro ligands increases, and it is tentatively suggested that this results from the reduced interaction of the SOMO of the radical and the LUMO of the alkene or alkyl bromide. All the radicals BunSnCl3-n (n = 0-3) react with biacetyl to show the e.s.r. spectra of the tin derivatives of butane-2,3-semidione, BunCl3-nSnOCMeCMeO., and these radicals have also been generated by a variety of other methods. Below about +20��C, when n = 0 or 1, the adducts Cl3SnOCMeCMeO. and BuCl2-SnOCMeCMeO. show hyperfine coupling by two non-equivalent methyl groups and one unique chlorine atom; above +20��C, the methyl groups become magnetically equivalent, and coupling is by more than one chlorine atom. (Chemical Equation Presented) This is interpreted to imply that the adducts have the structures (A) and (B) respectively, in which the ligands about the tin are approximately trigonal bipyramidal, and, at low temperatures, hyperfine coupling is by the apical chlorine atom; at higher temperatures, positional exchange between the ligands confers on the radicals C2v symmetry on the e.s.r. time scale. When n = 2, the two methyl groups are non-equivalent from -50 to 0��C, with no hyperfine coupling from chlorine, probably implying the static monodentate structure (C). When n = 3, two radicals have been identified. The first, with a spectrum which is a regular binomial septet, is thought to be the rapidly fluxional cis-monodentate compound (D). The second, which displays a septet spectrum with a severe alternating line width effect is thought to be the more slowly fluxional trans-compound (E).