Abstract: The values of the fluorescence quenching rate constants k_q, for 9, 10-dicyanoanthra-cene (DCA) and 9-cyanoanthracene (CNA) in different solvents by a series of compounds were determined and the exciplex emission was observed for the DCA/2, 5-dime-thylfuran system. A plot of k_q vs. calculated free energy change AG gave a very good correlation with the theoretical line calculated by Rehm and Weller for an electron-transfer process. Solvent polarity and solvent viscosity has great influence upon fluorescence quenching reactions: For strong electron donors the main factor which influences the k_q, values is the solvent viscosity and the k_q values of weak electron donors depend strongly on solvent polarity. The k_q of oxygen molecule coincide with the solvent diffusion rate constants k_diff.