The absolute value of refractive index and its variation with temperature of benzene and chloroform at He‐Ne laser wavelength 6328 A˚ are measured. The measurements are carried out using laser interferometric technique. This technique is based on the shift of interference pattern when rotating the sample in one arm of a newly developed Mach–Zehnder interferometer. The refractive index shows a linear relationship with temperature in the range of interest. The thermal refractive index coefficients of benzene and chloroform are all negative. Applying Maxwell’s theory and Lorentz–Lorenz function, the obtained results are used for calculating the optical permittivity (or dielectric susceptibility) as a function of temperature and many other important related physical constants. ©1995 American Institute of Physics.