A new method is proposed to demonstrate the existence of a true polarization threshold field in materials that demonstrate ferrielectric properties. The detection of small changes in the Bragg diffraction angles of a diffracter x‐ray beam has been used to show the existence of a true polarization threshold field in one such material, namely bismuth titanate (Bi4Ti3O12). It is shown that the x‐ray diffraction method effectively establishes the polarization threshold field of bismuth titanate at 3 kV/cm. This value of polarization threshold field agrees with that found previously using direct electrical methods. Prior to this work, the demonstration of a true polarization threshold field has been shown only by direct electrical methods.