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Oxidized cytochrome c oxidase can now be prepared either to react rapidly (fast) or to react slowly (slow) with cyanide [Baker, G.M., Noguchi, M., & Palmer, G. (1987) J. Biol. Chem. 262, 595-604]. Slow oxidized cytochrome c oxidase is also characterized by an integer spin g = 12 EPR signal which is absent in the fast form. The saturation magnetization of two samples of both forms of cytochrome oxidase has been studied at four applied magnetic fields (0.3125, 1.25, 2.5, and 5.0 T) over a temperature range from 2 to 200 K using a superconducting susceptometer. The saturation magnetization data of the two preparations are readily distinguished. The data for the coupled cytochrome a3: CuB site of both preparations are most simply interpreted as exhibiting S = 2 paramagnetism with D = +13 cm-1 for the fast and D = -7 cm-1 for the slow forms, respectively. However, there is some indication that the fast form is a mixture of both S = 2 and S = 1 paramagnetic species while the slow form is only S = 2.