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The dissociation constant Kdof the photosystem I (PSI):ferredoxin complex has been measured by backscattering interferometry (BSI) with cyanobacterial PSI (350 kDa) and ferredoxin (10.5 kDa). The BSI signal, consisting of shifts for interference fringes resulting from a change in refractive index due to complex formation, was monitored as ferredoxin concentration was titrated. Kdvalues of 0.14−0.38 μM were obtained with wild-type PSI whereas no complex was detectable with a PSI mutant containing a single mutation (R39Q) in the PsaE extrinsic subunit. These results are in quantitative agreement with previous functional determinations consisting in the detection of fast electron transfer within the complex. They provide evidence that the main contribution for the high affinity binding of ferredoxin to PSI is due to a single region of PsaE comprising arginine 39. They do not support the existence of a secondary binding site that could have escaped functional detection.