The refractive index is measured by means of a Mach-Zehnder interferometer.The measured values of the refractive index and the concentration CFs are compared with values obtained by two different theoretical predictions, the well-known Lorentz-Lorenz formula and an alternative one proposed by Looyenga.The refractive index of the THN-C12 mixture is found to vary both over pres sure and concentration, and the concentration CF is found to exponentially decrease as the pressure is increased.