A simple method is presented for calculating the oxidation state of Sm in complexes where Sm is bonded only to O ligands. A total of 88 SmOn fragments with n = 4-12 were retrieved from the Cambridge Structural Database and were analyzed using the bond valence sum (BVS) method. New R-0 values for Sm(II)-O of 2.116(21) Angstrom and for Sm(III)-O of 2.055(13) Angstrom were derived. The average R-0 value of 2.086 Angstrom gives a good approximation of the oxidation state of the Sm ion, either +2 or +3, from the observed distances without any assumptions. The Sm-O distances for +2 and +3 complexes with coordination numbers of 4-11 are tabulated and reflect the requirement that the BVS must equal the oxidation state. The distances for CN = 12 were not included because of problems with the reported crystal structures. Several X-ray structure determinations where the BVS and the oxidation state did not agree are discussed.