A group theoretical method is developed to treat the ground and excited electronic states of multielectronic polynuclear mixed-valence systems of the general form d n - -d n -d n + 1 - -d n + 1 and f n - -f n -f n + 1 - -f n + 1 . Effects arising from electronic degeneracy, spin-orbit interactions and reduced site symmetry are explicitly taken into account. The versatility of the technique is demonstrated by applying it to a series of trimeric mixed-valence clusters.