Several toughening and crosslinking modifiers were tested in two epoxy resin systems based on the diglycidyl ether of bisphenol A (DGEBA) with the objective to improve the critical stress intensity factor K-IC and the glass transition temperature (T-g) simultaneously. An amine hardener (isophorone diamine (IPD)) and a homopolymerization initiator (1-ethyl-3-methylimidazolium acetate (EMIM Ac)) were used as curing agents. The highest effect on the K-IC value of the resin system DGEBA/IPD (K-IC = 0.72 MPa1/2; T-g = 164 degrees C) was achieved with the dendric polymer Boltorn P501 (10 wt%), but it decreased the T-g (K-IC = 1.39 MPa1/2; T-g = 136 degrees C). A high toughening effect with a low decrease of T-g was achieved with a combination of a self-organized block copolymer (Nanostrength M22N) and silica nanoparticles (Nanopox F400) (K-IC =1.15 MPa1/2; T-g =157 degrees C). The K-IC value of the resin system DGEBA/EMIM Ac was improved from 0.44 to 0.66 MPa1/2. An improvement of both, the thermal and mechanical properties was established for a combination of a poly(tetrahydrofuran) as toughening modifier (PolyTHF2000) with the post-crosslinking modifier diethylphosphite (DEP) in the resin system DGEBA/IPD (K-IC = 0.86 MPa1/2; T-g = 180 degrees C). A system with chemical linkages between both modifiers was investigated for comparison but yielded inferior results. POLYM. ENG. SCI., 59:86-95, 2019. (c) 2018 Society of Plastics Engineers