The differential thermal analysis (DTA) and the differential scanning calorimetry (DSC) were carried out with various two-component mixtures of organic materials prepared by mechanical mixing, by solidification after fusion and by evaporation of the solution of components. As was expected, when the components did not form a molecular compound, no distinct difference in the thermograms were observed between the kinds of samples except that in the ones for the simple physical mixture, additional small peaks due to polymorphic transition appeared occasionally. On the contrary, the method of sample preparation exerted marked influence on the result, if the molecular compound was formed between components. Usually, the evaporated mixture solidifies in the state of stable equilibrium ; therefore, the DTA and DSC curves were the same as that for the mixture of the isolated compound and one of the components. In the case of the physical mixture which is in the metastable state, the curves showed characteristic peaks of the metastable fusion (endothermic), and of the subsequent compound formation and solidification (exothermic). Thus, by applying DTA or DSC to the physical mixture, a simple method was developed for detecting the molecular compound between organic materials. Also, the heat of formation of the molecular compound could be obtained by reducing the total area of the DSC peaks for the isolated compound from that for the corresponding physical mixture.