The magnetic field dependence of qce, the cage escape yield of radicals generated by photoelectron transfer between Ru(bpy)3zt and methylviologen (MV++) was investigated for a series of solvent mixtures of increasing viscosity, obtained by adding ethylene glycol (EGLY) to H20/ACN. From the experimental results the absolute values of the rate parameters characterizing the primary radical ion pair (kce for cage escape, zs for spin relaxation, and b e t for spin-allowed backward electron transfer) have been obtained. Particularly noteworthy is the strong viscosity dependence of b e t , which can be assigned to dielectric solvent relaxation as the rate limiting step of backward electron transfer in the EGLY containing solvents.