Thermal effects accompanying chemical reactions and phase changes in building materials at elevated temperatures are analyzed using a model of an n-component chemically reacting mixture. The concept of an effective specific heat, c(ef), is introduced which accounts for these two effects, and a nonadiabatic calorimeter is used in measuring c(ef) of selected building materials. The analysis of different types of building materials leads to an identification of several characteristic c(ef)(T) functions. For compact, thermally stable materials such as concrete, the effective specific heat increases monotonously with temperature, which is analogous to homogeneous materials, e.g., metals. The behavior of light macroporous materials varies depending on the type of material and it is not possible to generalize about the character of the c(ef)(T) function. One characteristic group is represented by materials containing organic compounds which exhibit dramatic changes in the effective specific heat in the temperature range similar to 300-600 degrees C when the burning temperature is reached. The effective specific heat of materials undergoing structural changes and/or chemical reactions due to increasing temperature, e.g., porous concrete, Porfix, etc., may either decrease with temperature or have an "oscillatory" character.