The spin dynamics of the duroquinone anion radical generated by photoinduced electron transfer reactions from triplet erythrosin B to duroquinone has been studied by using transient absorption and pulsed FT-EPR spectroscopy. Triplet exciplex formation as the reaction intermediate is verified by the observation of spin-orbit coupling induced electron spin polarization. The kinetic parameters for exciplex formation and the intrinsic enhancement factors of electron spin polarization are determined in various alcoholic solvents. The zero-field splitting constants of the triplet exciplex are determined by the analysis of the solvent viscosity dependence of the enhancement factors of the electron spin polarization. (C) 2002 Elsevier Science B.V. All rights reserved.