Magnetic-field-modulated exciplex luminescence in different solvent media has been studied to verify the solvent microheterogeneity effect. In particular, mixtures of a low-polarity component, such as dioxane and tetrahydrofuran, and a highly polar component, such as water, have been used as media where the microheterogeneity has been further modulated by addition of salt and variation of temperature. The results are analysed in light of several proposed models for the solvent effect on MFE’s, but no model is found to be completely satisfactory. It has been suggested that multiple intermediate states, instead of a single intermediate state, are formed during the electron-transfer process.