Specific refractive index increments of polyisoprenes with high cis-1,4 content have been determined in six solvents at 20� and λ 546 nm. A linear relationship was found between the refractive index increment dn/dc = n and the refractive index of the solvent nl : N45620 = 1.687 - 1.105n1 The results are compared with the best values reported for natural rubber which are usually employed for polyisoprene in light scattering measurements.