The triplet state radical-ion pair (RIP) formed in the photolysis of xanthone (Xn) and N,N-diethylaniline (DEA) in a highly viscous mixtures of 2-propanol and cyclohexanol was studied by time-resolved ESR. As the viscosity of the mixed solution increases, the spectrum reveals a magnetic dipole-dipole interaction in the triplet state of the RIP. Immediately after laser photolysis, the spin-polarized RIP spectrum exhibits magnetophotoselection (MPS). This suggests that the electron transfer (ET) reaction is faster than the longitudinal relaxation of the excited triplet state of Xn ((3)Xn*) or much faster than the tumbling motion of (3)Xn*. The former mechanism is likely under the conditions employed. Indeed, a huge RIP-cored aggregation is quickly formed with solvent molecules which obstruct the free revolution of the RIP. The MPS spectra also indicate that only the molecules closely located react by the solvent-mediated ET.