Embrittlement of duplex stainless steels after aging below 500 °C is a well documented phenomenon, usually attributed to the unmixing of the ferritic Fe-Cr solid solution, either by spinodal decomposition or by nucleation and growth of Cr-rich α′ phase, depending on aging temperature. Additional precipitation taking place during aging, like NiSiMo-rich G phase or Cu-ε, are also known to participate to embrittlement, making the composition dependence of embrittlement kinetics even more intricate. In this study, long term aging treatments, up to 2 years at intermediate temperature (250–400 °C), have been performed for various duplex stainless steels grades containing different alloying contents in Cr, Mo, Si, Ni and N. Impact toughness has been measured after each aging condition, and an apparent activation energy could be deduced for the various grades. This quantity is a good indicator of embrittlement resistance. Fine microstructural characterizations have been performed in the ferrite: composition before aging was determined by EPMA, and precipitation state before and after aging was observed on TEM. The influence of alloying elements on the embrittlement phenomenon is analyzed on the basis of these results and of driving force calculations performed on Thermo-Calc®.