This work deals with the toughening effect of flaky WS2 and fullerene-like WS2 (IF-WS2) nanoparticles on epoxy with varying network properties. Reducing the amount of curing agent resulted in decreased crosslink density as measured by dynamic-mechanic analysis and double-quantum nuclear magnetic resonance spectroscopy. Although that lead to moderate changes in the epoxy's tensile properties, its fracture toughness dropped drastically, probably due to an increased defect fraction. IF-WS2 could be dispersed significantly more effectively within epoxy resin than flaky WS2, possibly due to its spherical shape, but caused less toughening. IF-WS2 tended to debond from the epoxy, while flaky WS2 introduced more secondary cracks. Both increased the fracture toughness of the (brittle) substoichiometric, but not that of the (tough) stoichiometric epoxy, possibly due to their interaction with molecular defects. Irrespective of which mechanism resulted in the toughening effect, its effectiveness depended strongly on the epoxy matrix. (C) 2016 Wiley Periodicals, Inc.