A simple method for calculating the oxidation state of Cr in complexes containing only Cr-O bonds is presented. A total of 242 CrOn, fragments with n = 3-6 were retrieved from the Cambridge Structural Database (CSD) and, together with the data for K3CrO8, were analyzed using the bond valence sum method. New R-0 values for Cr(II) of 1.739(21) Angstrom, Cr(III) of 1.708(7) Angstrom, Cr(V) of 1.762(14) Angstrom, and Cr(VI) of 1.793(7) Angstrom were derived. An average Ro value of 1.724 Angstrom for Cr-O reproduces the oxidation state of 96 of the 110 Cr(II), Cr(III), and Cr(IV) CrOn complexes (n 3-6) and that of K3CrO8 within 0.30 valence units. The crystal structure of K3CrO8 was redetermined at 173 K to provide accurate data for a Cr complex with both high oxidation state and coordination number. Potassium tetraperoxochromate(V), K3CrO8, is tetragonal, Space group (I) over bar 42m, a = b = 6.6940(3) Angstrom, c = 7.7536(5) Angstrom, Z = 2. The difficulties with fitting the observed valence for Cr(V) and Cr(VI) complexes with coordination numbers 4 and 5 are discussed. The use of bond valence sums in gaining chemical insight into Cr complexes with noninnocent ligands and in establishing oxidation states in Cr clusters is presented. An analysis of the Cr-O bond distances used in the calculations shows a large range of values that can be understood in terms of the bond valence sum calculation.