A new cluster compound [WOS 3 Cu 3 I(py) 5 ] (I) was synthesized by substitution reactions of (Et 4 N) 3 [WOS 3 -Cu 3 I 4 ] 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) Å, V ) 6424(3) Å 3 , and Z ) 8. The cluster has a nest-shaped skeleton. The nonlinear optical properties of both I and [MoOS 3 Cu 3 I(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 × 10 -9 cm 2 W -1 M -1 ), while compound II exhibits a self-defocusing property (n 2 ) -1.3 × 10 -10 cm 2 W -1 M -1 ). Both compounds show reverse saturable absorption (R 2 ) 1.5 × 10 -4 and 2.5 × 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.