A new method for increasing fracture toughness of brittle thermoplastic-modified thermosets by using triblock copolymers has been successfully investigated. The selected systems were polyphenylene ether (PPE)- and polyetherimide (PEI)-modified epoxy networks. Our choice was restricted to available commercial copolymers possibly with some chemical modifications. PPE presents the substantial advantage of having a negative enthalpy of mixing with poly styrene. The maleic anhydride-modi fled poly(styrene-b-ethylene-co-butene-b-styrene) triblock copolymer, containing an immiscible elastomer central block, was then selected. The reactivity of succinic anhydride functions towards primary amines was used to graft on ethylene-co-butene blocks, chains which are miscible or able to react with the growing epoxy network. The two problems encountered with PEI is that PEI is not miscible with any other polymer and that a commercial triblock with a PEI block does not actually exist. The only copolymer commercially available is a poly(etherimide-b-dimethylsiloxane) segmented copolymer, with elastomer segments which are known to be strongly immiscible with any components. In order to obtain the characteristics of the required compatibilizer, the poly(caprolactone-b-dimethylsiloxane-b-caprolactone) triblock copolymer was associated since (a) the polydimethylsiloxane elastomer central block is chemically identical to the elastomer segment of the previous copolymer, and (b) the poly caprolactone blocks are totally miscible with epoxy. For both thermoplastic-modified epoxy networks, spectacular mechanical reinforcements were measured with only 10%b.w. thermoplastic as a result of interfacial activities of selected compatiblizing systems, with a relative enhancement of fracture toughness close to 50% with around 1% of copolymer. The positive effects on mechanical properties always result from the same causes: large decrease of the particle size (submicron size) and formation of a copolymer-rich interphase characterized by a micromechanical transition in mechanical spectroscopy.