The in-situ FT-IRRAS (Fourier transform infrared reflection absorption spectroscopy) technique has been applied to measure the nu(CO) values of the four oxidation states of the multistep redox system [Ru-3(mu(3)-O)(mu-CH3COO)(6)(CO)(pyridine)(2)](n+), in acetonitrile, of which only the n = 0 state has been isolated as a stable solid. The nu(CO) values for the n = -1, 0, +1, and +2 states are 1900, 1940, 2055, and 2110 cm(-1), respectively, indicating the approximate oxidation states of the Ru ion with CO are II, II, III, and III, respectively.