A method of measuring optical phase differences caused by the presence of optical path differences between two coherent light beams is reported. A polarization technique is used and the phase differences are detected as rotation angles of the plane of polarization. The sensitivity of the method is better than 10 -4λ and the stability is 3 X 10 -4λ per minute. These limitations result from thermal fluctuations of the order of 10 -3°C. A higher sensitivity of about 10 -5λ, or better, is expected if the effect due to thermal fluctuations can be reduced.