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Electron transfer is an essential process in all biological systems. The kinetics of electron transfer reactions between small inorganic complexes and a variety of metalloproteins have been extensively studied1–5. Recent studies on the electron transfer reactions of plastocyanin, an essential component of the photosynthetic electron transport chain, have shown6–9that inorganic redox reagents such as the hexacyanoferrate(III) ion (Fe(CN)63−6) and the tris(1,10-phenanthroline)cobalt(III) ion (Co(1,10-phen)33+) form discrete complexes with the protein before the electron transfer step. Here, we have used high resolution nuclear magnetic resonance spectroscopy (NMR) to study the interaction of plastocyanins from French bean (Phaseolus vulgaris) and cucumber (Cucumis sativm) with the chromium(III) analogues of these redox reagents, K3Cr(CN)6and Cr(1,10-phen)3(ClO4)3. The binding of these complexes to plastocyanin is highly specific and we identify two binding sites, separated by about 15 Å, which are apparently determined primarily by electrostatic interactions. Two distinct electron transfer pathways to and from the copper atom are indicated.