Donor molecule bis(ethylenedithio)tetrathiofulvalene (BEDT-TTF, or short for ET, see Scheme 1) constitutes a member of an important class of molecular organic conductors. Using the electrochemical method, we have successfully synthesized ET 2 Hg(SCN) 2 Cl. The compound was characterized by x-ray powder diffraction and IR spectrum methods. The linear refractive index of its acetonitrile solution in the concentration of 5.357×10 -5 mol/L as measured at 20 °C using V-prism refractometer at four different wavelengths, and the results were fitted with the three-term Sellmeier dispersion function. The molecular second hyperpolarizability y of ET 2 Hg(SCN) 2 Cl was determined by z-scan technique. A value of be 6.64×10 -32 esu was obtained.