The formation of secondary phases during isothermal treatments in the range 750-1000 degrees C and continuous cooling in 2205, 2507, 2304, and 2101 duplex stainless steels have been investigated. For all the steels herein considered, the Thermocalc calculations indicate the sigma and chi-phase precipitation which is confirmed by the experimental results only for the 2205 and 2507 grades. On the contrary, the secondary phases are very rarely observed after both isothermal aging (up to 750 h) and/or continuous cooling tests in the 2304 and 2101 Cr-Mn grades. This behavior could be justified by the different ferrite and austenite phase stability in the four grades, in the same temperature range of the sigma and chi precipitation, because these differences affect the dangerous phases precipitation mechanism and kinetic.