The use of the bond valence sum in Fe-O complexes is discussed using data from the Cambridge Structural Database. The oxidation state of Fe in complexes containing Fe bonded only to O can be calculated by Sigma(i) exp[(R-0-r(i))/b], where r(i) is the observed Fe-O-t bond length, R-0 is a constant = 1.745 Angstrom, and (b)over dot is taken to be 0.37. The R-0 value can be viewed as a bond length of unit valence. Deviations from an integer value indicate a problem with the crystal structure or electron delocalization in multinuclear complexes. The oxidation-state-independent R-0 value of 1.745 Angstrom was calculated using 297 FeOn complexes with n = 3-8. An R-0 value of 1.714 Angstrom for Fe(II) was determined using 74 complexes, and a value of 1.750 Angstrom was determined for Fe(III) using 223 complexes.