Mach–Zehnder interferometry was used to measure the Pockels effect in a poled electro‐optic polymer thin film with a coplanar electrode structure. The beam at the sample arm of the Mach–Zehnder interferometer passed through a polymer thin film which had been spin‐coated on top of a clear gap between two electrodes patterned on an optical substrate. This unique optical geometry enabled the Pockels coefficients of the poled electro‐optic polymer film in the directions of the ordinary and the extraordinary optic axes to be determined independently. As an example, the tensor ratior33/r13for a stilbene‐dye‐doped polyimide guest/host polymer film was determined experimentally; the ratio turned out to be 4.6, which was higher than the value of 3 predicted by the thermodynamic model. ©1996 American Institute of Physics.