Refractive indices of ternary mixtures formed by cyclohexane, toluene, and methanol are reported over a wide range of mixture compositions. This mixture exhibits a miscibility gap and for a long time has attracted the attention of scientists from different areas of research. All measurements of refractive indices have been conducted at 298.15 K and atmospheric pressure using two light sources: one in the visible (lambda = 670 nm) and the other in the infrared (lambda = 925 nm) spectrum. The performance of the Lorentz-Lorenz mixing rule that is commonly used in modeling optical constants is examined. The concentration derivatives of the refractive index are an important parameter, as they are required for different experimental techniques and, particularly, for the measurements of the thermodiffusion coefficient. These derivatives have been determined from the experimental data on refractive indices. The mixture compositions, where these two wavelengths are applicable for the measurements of mass transport coefficients, are determined and discussed.