Ductile irons of the type of Si-Mo are characterized by increased resistance to long-term influence of high temperatures and cyclic temperature changes. They are mainly used in castings of combustion engine exhaust piping and other castings utilized at temperatures of up to 850°C. The aim of the study is to verify the mechanical properties of non-alloyed cast iron EN CSN GJS 450, SiMo4-0.5 and SiMo5-1 ductile irons at temperatures of 700 to 800°C, and the extent of their superficial oxidation after longterm annealing at a temperature of 900°C. Via chemical microanalysis the composition of oxidation products in the surface layer was evaluated.