The effect of a simple modification of the form of the Gibbs free energy that is normally used to describe the presence of Frenkel defects in crystals with the fluorite structure is examined. It is found that interactions among the high concentration of defects, present in these materials at high temperatures, can provide an adequate explanation of the observed specific heats and equilibrium concentrations of defects in PbF2, SrCl2, and BaF2. It is found that in order to obtain satisfactory quantitative agreement between the calculated and measured defect concentrations and specific heats, it is necessary to presume Frenkel-defect formation entropies that are in serious disagreement with values obtained from most electrical-conductivity measurements, but consistent with several other values. We also compare calculated values of several other quantities to the experiment. © 1981 The American Physical Society.