Abstract A comparison of specific heat, ultrasonic attenuation, and soft-mode linewidth divergences is presented for several ferrodistortive and antiferro-distortive phase transitions, including BaMnF4, TGS, KDP, SrTiO3 and KMnF3. In the cases of BaMnF4, KDP and TGS the specific heat divergences exhibit power-law form, and the observed exponent (approximately ½) does not permit easy distinction between intrinsic and extrinsic (defect) theories, since these three crystals are all near tricritical points, for which the predicted exponent is the same (−0.5) in mean-field theory and in the defect theory of Levanyuk, Osipov, Sigov and Sobyanin. However, ultrasonic attenuation and soft-mode linewidth behavior strongly favor the defect interpretation. The specific heat behavior in BaMnF4 satisfies the predictions of the Pippard-Janovec relationship.