Magnetokinetic effects were observed between 0 and 8 T in electron-transfer reactions of Ru(bipy)(3)(3+) with ferrous cytochrome c, Fe(bipy)(3)(2+), Fe(Me4-[14]1,3,8,10-tetraene-1,4,8,11-N-4)(solvent)(2)(2+), and RU(NH3)(6)(2+). The effects have been related to the adiabatic character of the reactions and rationalized in terms of a mechanism that incorporates the spin-orbit coupling, hyperfine-coupling, and Zeeman mechanism in the expression of the reaction rate constant.