A study of viscosity and temperature effects on the rate of back electron transfer (BET) within an exciplex (9,10-dicyanoanthracene/N,N-dimethylaniline) with a strong charge transfer character in six non-polar solvents is reported. The extent of charge transfer has been estimated from the solvatochromic and thermochromic shifts of the fluorescence. Conformational changes are a prerequisite to the BET. In non-viscous solvents, where they are much faster than the ET step itself, the observed rate can be explained within the theory of non-adiabatic ET reactions, while in more viscous solvents, a time-dependent electronic coupling constant V has been introduced. In decalin and butylbenzene, a transition from a "solvent independent" to a "solvent controlled" non-adiabatic regime is observed.