Duplex stainless steels constitute a famous group of the stainless steels. They consist of mainly ferritic and austenitic phases. The ratio between these phases is regulated by numerous alloying elements and low content of carbon. Because of this strong alloying a lot of metallurgical processes can take place at high temperature, e.g. the decomposition of ferritic phase into σ-phase and secondary austenite. In this paper the changes of amounts of these three phases are studied by automated EBSD, saturation magnetization measurement, XRD-analysis and micro-hardness test. Similar results were obtained by the different applied measurements for the change of phase ratios due to the isothermal heat treatment.