Novel toughened but highly cross-linked epoxy thermosets are presented as new liquid adhesives. Linear polyols of oligomeric poly(E-caprolactone) and malic acid containing OH-groups along the linear polyester chain were covalently integrated in a cationically polymerized, cycloaliphatic epoxy resin. The clear and transparent polymers were characterized mechanically. High mechanical strength and significantly increased fracture energy were received without lowering the high thermal stability of the thermosetting epoxy. Lap shear tests of these modified adhesives showed higher tensile shear strength on aluminum compared to the unmodified epoxy resin.