The reaction between the heteropolyanion Co II W 12 O 40 6؊ , in the form of its (Bu 4 N) 6 (2) or K 2 (Bu 4 N) 4 salt (3), and NO 2 ϩ BF 4 ؊ has been studied kinetically in acetonitrile or nitromethane at 26 ЊC. In acetonitrile, the rate constants of 2 and 3 are almost identical, 0.097 and 0.073 dm 3 mol ؊1 s ؊1 , whereas in nitromethane 2 reacts >10 2 times faster, k = 13 dm 3 mol ؊1 s ؊1 (3 has not been studied in nitromethane for solubility reasons). A second inorganic species, (Ph 4 P) 2 Os IV Cl 6 , has also been similarly investigated, for which k = 1.4 × 10 3 dm 3 mol ؊1 s ؊1 in acetonitrile. The rate constants of the reaction of 2 (3) with NO 2 ϩ could be correctly estimated by the Marcus theory of outer-sphere electron transfer, using the exceptionally high value of 140 kcal mol ؊1 of the reorganization energy of the NO 2 ϩ /NO 2 homoexchange reaction which earlier had been estimated theoretically. Thus it seems that this value is supported by experimental evidence.The reversible potential of NO 2 ϩ /NO 2 in nitromethane has been determined to be 0.95 V vs. the ferricinium/ferrocene couple, thus confirming previously reported values in the vicinity of 1.0 V, but in disagreement with a higher value, 1.62 V.