The article presents crystallization process of silicon molybdenum ductile cast iron (SiMo). The alloy with 5% silicon content and with variable amounts of Mo in a range of 0-1% was chosen for the research. The carbon content in the analysed alloys did not exceed 3,1%. The studies of crystallization process were based on thermal - derivative analysis (TDA). Chemical composition of all examined samples was analysed with the use of LECO spectrometer. Additionally, the carbon and the sulphur content was determined basing on carbon and sulphur LECO analyser. For metallographic examination, the scanning electron microscopy (SEM) with EDS analyser was used. Disclosed phases have been also tested with the use of X-ray diffraction. The results allowed the description of crystallization processes of silicon molybdenum ductile cast iron using thermal - derivative analysis (TDA). Conducted studies did not allow for the clear identification of all complex phases containing molybdenum, occurring at the grain boundaries. Therefore, the further stages of the research could include the use of a transmission electron microscope to specify the description of complex compounds present in the alloy.