Density, refractive index, and their temperature derivatives are measured over a wide range of concentration, temperature, and wavelength in the partially miscible binary liquid 2,6-lutidine-water. The expected anomalies at the critical concentration are within the relative precision of the measurements (a few 10(-6)) . The Lorentz-Lorenz formula is compared with the data in the whole concentration range. It remains valid within the regular correction factors. We present fitting functions in temperature, concentration, and wavelength.