Mach-Zehnder optical interferometry with phase shifting is applied to interdiffusion experiments in binary liquid mixtures. A new method of analysis is presented and applied to measurements on two test systems, NaCl/water and 1-butanol/water at atmospheric pressure and room temperature, and to numerical solutions of the diffusion equation. The precision and accuracy of the technique is found to be 0.6% for the NaCL/water system and 1.4% for the 1-butanol/water system. The analysis is based on the assumption of ideal boundary conditions and expansion in Hermite polynomials of the reduced phase of the light passing through the sample, phi(eta), around the solution to the ideal diffusion equation. The numerical calculations show that an approximation, D-2mH2, to the reduced second moment of the derivative phi’(eta) is a better estimate for the diffusion coefficient than the reduced height area ratio, D-A, obtainable by Rayleigh and Gouy interferometry. The analysis yields a measure of a single-experiment accuracy, The main source of error is not in the data acquisition or analysis but rather in the way the initial concentration profile is established. We also report new experimental interdiffusion coefficients of methane/n-decane, for mole fraction of methane 0.098, temperatures 30, 90, and 150 degrees C, and pressures 20, 40, and 60 MPa. These diffusion coefficients have an estimated accuracy of between 0.7 and 2%. An extension of the model of Assael et al. is fitted to the data to within experimental accuracy.