N.m.r. coupling constants 1J(M–P) are reported for complexes of WIV, PtI, and PtIV. Results for 1J(Pt–P) in the complexes [PtCl4–n(R3P)n](n-2)+ and [PtCl6–n(R3P)n](n–2)+(n= 1–3) are discussed in terms of the M.O. expression for the Fermi contact interaction. Interpretation of coupling constants in terms of the s-orbital bond order is supported by the correlation of bond lengths with 1J(Pt–P) for PtII complexes and by the similarity of trends in bond lengths and M–P coupling constants in mer-tris(phosphine) complexes of WIV, RhI, RhIII, PtII, and PtIV. It is also shown that, for both 1J(Pt–P) and 2J(PtCH), ratios of coupling constants in related PtIV and PtII complexes reflect bond-length differences in the two oxidation states.