For the prediction of the transient behavior of thermal nodes which are interacting within a Thermal Mathematical Model (TMM) it is necessary to know the heat capacity of each node. For instance this is actual for components of opto-electronic devices for space exploration. Other assignment is to define the thermal properties of new structure materials and their combinations.Often the base for the correction of the TMM is the comparison of the calculated node temperatures with the node temperatures measured on a Thermal Engineering Model (TEM) during a Thermal Vacuum Test. The TEM has to be very similar to the flight hardware from the thermal point of view. But very expensive flight components are replaced in the TEM by thermal equivalent dummies. This makes it possible to use all components of the TEM for an unusual but simple experimental determination of their heat capacity as well.Survey of experimental methods of heat capacity definition intended for composed structural units has proposed to use two methods: the method of mix calorimeter and the method of evaporation of a calibrating liquid. The advantage of these methods is that the experimental effort is not high and the equipment for test realization is available in the most laboratories dealing with thermal problems of space borne equipment. The possibility to test objects with a complicated geometry is assumed as merit also. The first method is being applied for average heat capacity definition within a narrow temperature interval (ΔT = 5…15K), the second one - within a wide interval (ΔT = 50…150 K). The main measured parameter is the liquid temperature change and the liquid mass change correspondingly.