The nanoscale concentration fluctuation evolution due to phase separation (PS) and the corresponding mechanical property changes in two duplex stainless steels, 22Cr-5Ni (2205) and 25Cr-7N1 (2507), have been studied after aging at 325 degrees C for up to 6000 h. The nanostructure characterization is performed using small-angle neutron scattering (SANS) and the microstructure and fractography analyses, including observations on fracture surfaces and fracture cross-sections, are performed by scanning electron microscopy and electron backscatter diffraction. The results show that the kinetics of PS in grade 2507 is faster than that in grade 2205, leading to greater hardening and deterioration in toughness for grade 2507 as compared to grade 2205. The evolution of the nanostructure in the ferrite phase changes the deformation mode from the original ductile fracture to a quasi-cleavage type fracture where deformation twins form in the hardened ferrite. Delamination, grain fragmentation in ferrite and plastic slip deformation of the austenite are suggested to dissipate most of the energy absorbed by the crack during brittle fracture.