A new inorganic cluster (n-Bu(4)N)(4)[Mo8Cu12O8S24] was synthesized. It proves to be a superior molecular third-order nonlinear optical chromophore. Results from solution measurements of degenerate four-wave mixing and Z-scans at 532 nm show that the observed value of the third-order nonlinear molecular susceptibility, gamma, is 5.7 x 10(-28) esu with imaginary and real components, Im gamma and Re gamma, of 0.45 x 10(-28) esu and -5.7 x 10(-28) esu, respectively. The negative sign of Re gamma reveals that the cluster has an interesting optical self-defocusing property. The third-order susceptibility (chi s((3)) = 3 x 10(-6) esu, 532 nm) of the molecule in the solid state is extrapolated.