The development of bicontinuous morphologies in 10 wt% polysulfone (PSu)-epoxy (DGEBA)/anhydride (MTHPA) blends, was followed by optical and scanning electron microscopy. Blends cured at 80 degrees C revealed the formation of large epoxy-rich domains surrounded by a PSu-rich matrix, soon after the cloud point. Advancing the cure led to an increase in the volume fraction and the coalescence of epoxy-rich domains. A bicontinuous primary morpholcgy was thus generated. A secondary phase separation was observed in both primary phases from the very beginning of the phase-separation process. While spinodal demixing was clearly the mechanism by which the primary morphology was generated, nucleation-growth could be responsible of the secondary phase separation. Postcure steps produced a change in the composition of phases as revealed by DMA, and in the secondary morphology as observed by SEM. A postcure at 120 degrees C led to a single T-g at 115 degrees C with a small shoulder at higher temperatures. A postcure at 200 degrees C led to a T-g at 108 degrees C for the epoxy-rich phase and a T-g at 137 degrees C for the PSu-rich phase. The partial purification of the thermoplastic phase produced a significant enhancement of toughness. K-IC was increased from 0.65 MPa m(1/2) for the neat thermoset to 1.10 MPa m(1/2) for the blend postcured at 200 degrees C.