Rate constants k(ex) for the Os(CN)(6)(3-/4-), MO(CN)(8)(3-/4-) and W(CN)(8)(3-/4-) self-exchange reactions in aqueous solution have been measured by C-13 NMR as functions of pressure, temperature, and added electrolyte concentrations. Detailed interpretation of k(ex) values in the absence of added electrolytes is complicated by anioncation association, but k(ex) is strongly influenced by the nature of added cations (Li+ < Na+ < K+ < Rb+ < Et4N+ < Cs+ less than or equal to Me4N+) and is linearly dependent on their concentrations. For K+ as the counterion, Delta V-ex(double dagger) for Os(CN)(6)(3-/4-), Mo(CN)(8)(3-/4-), and W(CN)(8)(3-/4-) is + 19.0 +/- 0.9, + 14.7 +/- 0.6, and +22.5 +/- 1.1 cm(3) mol(-1), respectively; these values are completely inconsistent with a simple two-sphere Marcus-type model of the electron-transfer process, which predicts Delta V-ex(double dagger) on the order of -6 cm(3) mol(-1). With Et4N+ as the counterion, however, Delta V-ex(double dagger) for Mo(CN)(8)(3-/4-) is -8.2 +/- 0.6 cm(3) mol(-1), and for W(CN)(8)(3-/4-) With Me4N+ Delta V-ex(double dagger) is -7.4 +/- 0.5 cm(3) mol(-1). The results are interpreted iri terms of a mechanism which is outer-sphere as far as the cyanometalate ions are concerned but in which a partially desolvated cation bridges the reacting anions. In the case of alkali metal cations, removal of some (probably two) of the coordinated water molecules precedes electron transfer, whereas the tetraalkylammonium ions have no coordinated water. Anion-anion electron transfer is much more sensitive to cation catalysis than cation-cation exchange is to anion effects, probably because a positively charged bridge provides a much more favorable tunneling path for the negatively charged electron than would an anionic mediator.