Spin-trapping studies, using R′NO (R′= Bu^t or C₆HMe₄-2,3,5,6), have been carried out on: (i) various Pt⁰–alkyl halide (RX) systems, e.g.[P(PR″₃)_n](R″= Et or Ph, n= 3 or 4)–Mel; (ii) a number of Pt^II–sulphonyl or acyl halide reactions, e.g. cis-[PtMe₂(PtMe₂Ph)₂]–p-MeC₆H₄SO₂X; and (iii) several Rh^I– or Ir^I–alkyl halide additions. In most cases the appropriate nitroxyl spin adduct R(R′)N·O or R′(p-MeC₆H₄SO₂)N·O [but not R-(R‴CO)N·O] is observed by e.s.r. spectroscopy. In conjunction with appropriate control experiments, this leads to the unequivocal conclusion that free radicals are implicated in systems (i)(X = Cl, Br, or l) and (ii)(X = Cl or Br). By means of the nitrone PhCHN(O) Bu^t, a platinum(I) complex has been trapped during the course of the [Pt(C₂H₄)(PPh₃)₂]–Etl reaction; its formulation as [Pt{CH(Ph)N·O(Bu^t)}l(PPh₃)₂] is based on e.s.r. data. Trityl chloride adds to [Pt(PPh₃)₃][but not so rapidly to a rhodium(I) or iridium(I) substrate] to give Ph₃Ċ and [PtCl₂(PMe₂Ph)₂]; Ph₂CHBr and [Pt(PPh₃)₃] give (Ph₂CH)₂ as the principal organic product. Galvinoxyl inhibits the addition of p-MeC₆H₄SO₂Cl to [PtMe₂(PMe₂Ph)₂]. Azobis(isobutyronitrile) under photolysis catalyses the oxidative addition of PhBr to [Pt(PPh₃)₃]. Whereas the addition of Mel to [Pt(PPh₃)₃] in benzene leads exclusively to the 1 : 1 adduct, in tetrahydrofuran by far the major product is [Ptl₂(PPh₃)₂]. It is concluded that reactive halides RX add to a platinum(0) substrate via a geminate radical pair [Pt^IL_n(X)]+ R˙, whereas with less reactive halides, or in the sulphonyl halide–Pt^II addition, a radical-chain mechanism is operative.