The kinetics of the electron transfer reaction between [Cr(2,2′-dipy)3]2+ and some cobalt(III) complexes has been measured in the presence and absence of 4,4′-dipy. On the basis of a previous investigation of the formation of a mixed protonated complex of chromium(II), 2,2′-dipy and 4,4′-dipy, the data are interpreted in terms of reaction of this complex with cobalt(III). From the relative values of the kinetic parameters, a mechanism is suggested for the reaction of the 4,4′-dipy complex, involving and intramolecular electron transfer from chromium(II) to 4,4′-dipy, and a subsequent outer-sphere electron transfer from 4,4′-dipy to cobalt(III).