Using the methods of viscometry, SEM, DMA the effect of polyetherimide (PEI) on the properties of ED-20 epoxy oligomer during and after curing was studied. It was shown that the introduction of the modifier significantly affects the kinetics of curing process and the structure of epoxy-polyetherimide matrices. The introduction of PEI reduces the curing time of epoxy resin blends. With increasing temperature, the curing time, working life and gel time of all compositions are reduced by an order. In homogeneous epoxy/PEI compositions, phase separation occurs during curing. It was found that with an increase in the PEI concentration to 25 parts by mass, the matrix-dispersion type of structure with PEI-rich particles and epoxy-rich continuous phase changes to a complex bicontinuous structure. The Young modulus of the cured compositions differs by no more than 15%, and the glass transition temperature (T-g) of the matrix with the addition of PEI increases.