Novel Liquid rubbers based on bishydroxy-terminated oligo(butylmethacrylate) were used to toughen anhydride-cured epoxy resins. Concentration and molecular weight of the toughening agents were varied in order to examine the effects on important mechanical properties, such as toughness, strength, stiffness, and glass transition temperature. Experimental data show that telechelic methacrylates are suitable toughening agents for epoxies. The compatibility between resin and toughener can be adjusted by varying the molecular weight of the rubber. The best results are obtained by modifying an epoxy resin with 10 wt% of bishydroxy-terminated oligo(butylmethacrylate) of a molecular weight of 5000 g/mol. Fracture toughness K-lc increases by 150% accompanied by a decrease in modulus of only 11% and in strength of 16%, as compared to the corresponding properties of the neat resin. Due to an almost complete phase separation of the rubber upon curing, the glass transition temperature is scarcely affected.