He article discusses some provisions of phase transitions of the first and second order. The features of thecourse of phase transformations of the first kind during firing of monothermitic ceramic samples are shown. Asan example, experimentally obtained differential curves (temperature fields) are given during linear heating ofthese samples. On the differential curves, which clearly show two endothermic and one exothermic effects in therespective temperature ranges: 100-300; 550-900 and 900-1000 оC. It has been established that in the temperaturerange 550-850 оC, where the dehydration process occurs, which is accompanied by phase transformations of thefirst kind, the effective value of the thermal diffusivity and specific heat of the sample varies over a wide range.This is explained by the influence of the thermodynamics of the endothermic effect occurring in the sample. Comparing these results with the features of the course of phase transformations of the second kind, the followingimportant conclusion was made. During phase transformations of the second kind, indirect thermal effects takeplace, associated with a change in the physical, in particular, thermophysical properties of materials and, accordingly, the deviation of the temperature fields of the sample when it is heated or cooled.