Chemorheology of curing as well as the phase separation behavior of polysulfone (PSU)-modified diaminodiphenylmethane-cured diglycidylether of bisphenol-A epoxy mixtures have been studied using several techniques. The delay in polymerization for the modified mixtures with respect to that of the neat epoxy can be explained by dilution and viscosity effects. The immiscibility of these mixtures has been proved for various PSU contents and as a function of the precuring conditions used as well. The control of the generated morphologies can be performed only by varying the processing temperature. The thermal and dynamic viscoelastic behavior of the modified matrices has been examined and compared to the parent epoxy matrix. Finally, the mechanical properties, including fracture toughness, have been discussed in terms of the morphological behavior for an epoxy matrix modified with various amounts of PSU and for a 15 wt.% PSU-containing matrix precured at different temperatures.