Due to thermal ageing several precipitation and segregation processes occur in austenitic-ferritic superduplex stainless steel. During phase transformation processes mostly intermetallic compounds, and-due to the high solved nitrogen content-nitrides appear. These changes in microstructure influence the two original phases (the ferrite and austenite) in different way. Thermal ageing was performed on SAF 2507 steel in a broad thermal interval. Changes in microstructure were followed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The concentration of small particles was determined by successively decreasing of accelerating voltage, corrected by a factor, which was determined by large-area measurements.