A new cluster compound [WOS3Cu3I(py)(5)] (I) was synthesized by substitution reactions of (Et(4)N)(3)[WOS3Cu3I4] With pyridine and structurally determined by X-ray diffraction. It crystallizes in monoclinic, space group C2/c (No. 15), a = 24.341(1), b = 10.020(4), c = 26.64(1) Angstrom, V = 6424(3) Angstrom(3), and Z = 8. The cluster has a nest-shaped skeleton. The nonlinear optical properties of both I and [MoOS3Cu3I(py)(5)] (II) in acetonitrile solutions were measured using a Z-scan technique with 7-ns and 532-nm laser pulses. Compound I possesses a self-focusing property (n(2) = 1.6 x 10(-9) cm(2) W-1 M(-1)), while compound II exhibits a self-defocusing property (n(2) = -1.3 x 10(-10) cm(2) W-1 M(-1)). Both compounds show reverse saturable absorption (alpha(2) = 1.5 x 10(-4) and 2.5 x 10(-5) cm W-1 M(-1) for compounds I and II, respectively). We demonstrate that the nonlinear refraction of a mixture containing the two compounds can be altered from self-focusing, through a state where self-lensing disappears, to self-defocusing by varying the ratio of the two compounds.