A simple procedure is developed for the measurement of the differential quadratic electrooptic coefficient, R-33, by two-beam polarization (TBP) interferometery. It is shown that a TBP interferometer can be used for measuring the Kerr coefficient of a thin film with a strong Fabry-Perot effect. The measured values of the differential effective Kerr coefficient, R-33, of lead zirconate titanate 52/48 thin film lies inside the interval between -0.5 x 10(-18) m(2)/V-2 and +1.7 x 10(-18) m(2)/V-2 for the external DC field from - 160 kV/cm to 160 kV/cm, in agreement with the known data. The correlation between differential and commonly used electrooptic coefficients is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.