Methods and apparatus are provided for determining the refractive index of a downhole fluid. Two unalike crystals are provided having faces in contact with fluid in the fluid flow line of a borehole tool. The crystals are chosen to have different refractive indices and/or different angles of incidence, but to provide total internal reflection for light that is directed through the crystals to the crystal/fluid interface. The measured attenuations for each crystal are used in conjunction with the known refractive indices and angles of incidence of said crystals to determine the refractive index of the fluid.