NMR spectroscopy was used to study the variability of magnetic exchange parameters in [Fe(2)Q(2)](2+/+) clusters in spinach ferredoxin (Q = S, Se). The spectra of the Se-substituted protein are reported for the first time, and a theoretical model, applied to estimate the change in the value of the magnetic coupling constant in the case of S and Se, indicates a ca. 10% decrease of J in the oxidized and reduced core.