The refractive index increment (dn/dc) of a solution is a constant that indicates the variation of the refractive index with the solute concentration. It is used in the multi-angle light scattering technique to determine the concentration and the weight-average molar mass of polymers. It depends on many parameters, including the structure of the polymer, the solvent, the wavelength of the light and the temperature of the solution. To obtain accurate results, it is necessary to determine the dn/dc at the same condition as for the multi-angle light scattering measurement. In this work, the dn/dc at 633 nm of standard synthetic monodispere poly(cis-1,4-isoprene) (PI) and natural rubber (NR) solutions in THF were determined at 25 and 40°C using an Optilab DSP refractometer. The mean value of the dn/dc obtained for NR and PI was 0.13 mL/g. No significant difference between different types of samples and temperatures (25°C and 40°C) were observed. The nanoaggregates in NR solution that were not retained after filtration through 1 ?m filter had no effect on the dn/dc. The intercept value of the conformation plot increased in line with the dn/dc, but the Flory exponent remained unchanged.