The base line for a reaction peak of a DTA-curve is used to calculate the absorbed (endothermic) or emitted (exothermic) energy. Therefore, the correct base line construction is of vital importance. A DTA-curve in general represents the heat capacity of the measured material. The reaction energy is the area under the measured peak and the corresponded base line. The aim of the diploma thesis is to verify the construction of base lines in irreversible reactions of DTA-curves. For these purpose the two secondary hardening steels, S903 and the maraging steel Corrax, which exhibit complex precipitation reactions were investigated. Additional to the DTA-measurements dilatometer curves were recorded to separate different reactions occurring in the materials investigated. To characterise each reaction peak of the DTA-run and to verify the correctness of the base line atom probe and field ion microscopic investigations were performed. According to the received results, the precipitation reactions could be clearly defined and the precipitates were classified. The correctness of the base line construction could also be confirmed by the obtained experimental results.