The paper studies the effect of heat-resistant thermoplasts (polyetherimide and polysulfone) on the adhesion of the ED-20 epoxy oligomer cured with diaminodiphenylsulfone to the fibres, the viscosity and wetting ability of the modified epoxy oligomer, and also internal stresses arising on the adhesive – substrate interface during formation of adhesive joints. It is shown that introduction of the thermoplasts increases the viscosity by 1,5…2 orders at curing temperature (180°C), decreases its wetting ability, practically does not affect the change of wetting angle while curing, reduces internal stresses and increases the adhesive strength of the joints of ED-20 with fibres. Possible mechanisms of observed increase in the interfacial strength are discussed