We suggest an empirical method to determine the optimum refractive index (RI) of a poly-component particulate system. RI is the ratio of the velocity of light (of specified wavelength) in air, to its velocity in the substance under examination. It may also be defined as the sine of the angle of incidence divided by the sine of the angle of refraction, as light passes from air into the substance. Materials properties affect the procedure depending on particle strength, wettability, and particle refractive index. Theoretically calculated and empirically measured values are compared. The method yields more accurate refractive index values for composite particulate systems of two and three components than well-known values. We tested the method using calcite and alumina and observed RI values of 1.510 for 1:1 calcite, 1.715 for 1:1 alumina, and 1.765 for 1:1:1 alumina. These values are within the ranges typically observed for calcite (1.480-1.650) and alumina (1.6704-1.7986). The method is especially useful when measuring the particle size distribution using laser diffraction and scattering. (C) 2009 Elsevier B.V. All rights reserved.