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The formation constants K1and K2of the complexes Fe(NCS)2+and Fe(NCS)2+in dimethyl sulfoxide (DMSO) and the rate constants and the activation parameters of the electron transfer reaction between Fe(II) and Fe(III) in DMSO catalyzed by thiocyanate ion were measured. K1and K2are (8.4±0.9)×102m−1and (0.97±0.10)×102m−1at μ=0.1 mand 25°C, respectively. The apparent rate constant kappis nearly independent of acid concentrations and increases markedly with total thiocyanate ion concentration. The rate constants k1and k2of Fe2+-Fe(NCS)2+and Fe2+-Fe(NCS)2+systems are (2.24±0.35)×102m−1sec−1and (2.96±0.64)×103m−1sec−1at μ=0.2 mand 25°C, respectively. ΔH1\eweq=8.9±1.0 kcal/mol and ΔS1\eweq=−17±3 cal/deg·mol are obtained from the temperature dependence of kapp. From a comparison with the data of cases catalyzed by Cl−and SCN−ions in water and DMSO, the present electron transfer reaction is deduced to proceed probably viathe inner-sphere mechanism with the SCN−ion as a bridging ligand, similar to the Cl−ion-catalyzed reaction in DMSO.