This paper is a further study of ion binding to protein surfaces and builds on the studies of the binding of [Cr(CN)6]3− and [Fe(edta)(H2O)]− previously reported [Williams et al. (1982) FEBS Lett. 15, 293–299; Eley et al. (1982) Eur. J. Biochem. 124, 295–303]. In the present paper the binding of polyaminocarboxylate complexes of gadolinium have been studied. Eight ion‐binding sites have been identified on the surface of cytochrome c. These exhibit different binding specificities which, in some cases, are not fully understood. However it is clear that simple outer‐sphere interactions are not the sole determining factor for the association of metal ion complexes with proteins. The NMR paramagnetic difference spectrum method has been shown to be good at locating binding sites and revealing qualitative differences in their relative affinities for a range of complex types. However the use of relaxation probes is not a good method for the quantitative determination of binding constants; for this, isostructural shift probes must be sought.