H-1 NMR and NOE measurements have been performed on reduced HiPIP from Chromatium vinosum. 1D and 2D saturation transfer experiments have allowed a full correlation between the isotropically shifted signals of the reduced and oxidized species. The pairwise assignment of the cysteine geminal beta-CH2 protons is performed. This allows the relative evaluation of the expectation value [S(iz)] for each iron ion i as well as its temperature dependence. A theoretical approach has been able to account for upfield shifts of beta-CH2 cysteine protons in the oxidized species. The H-1 NMR data turn out to be a valuable benchmark for testing theoretical models for Fe4S4 clusters. The present model supports the existence of a mixed-valence pair with S = 9/2 ground state in the oxidized protein and is consistent with the presence of analogous mixed-valence pairs also in the reduced protein. The NMR data on the oxidized protein also show that, even at room temperature, electron delocalization mainly occurs within one particular Fe(II)-Fe(III) pair.