Reliable analysis of the functionality of metalloproteins demands a highly accurate description of both the redox state and geometry of the metal centre, not only in the isolated metalloprotein but also in the transient complex with its target. Here, we demonstrate that the transient interaction between soluble cytochrome c 6 and membrane‐embedded photosystem I involves subtle changes in the heme iron, as inferred by X‐ray absorption spectroscopy (XAS). A slight shift to lower energies of the absorption edge of Fe2+ in cytochrome c 6 is observed upon interaction with photosystem I. This work constitutes a novel application of XAS to the analysis of weak complexes in solution.