Epoxy thermosets are commonly formulated with an excess of epoxy resin to ensure complete reaction of co-reactive hardeners and to optimize performance. However, the degree to which the reaction of the excess epoxy resin contributes to the thermal and mechanical properties of the thermoset is incompletely understood. In this report, the preparation of controlled epoxy thermosets containing varying amounts of excess epoxy resin having essentially complete excess epoxy conversion is described. The extent of conversion was determined using a solid-state C-13 NMR method with enhanced resolution due to solvent swelling of the thermosets. This etherification reaction increases the crosslink density of the epoxy thermosets but uniquely affects the thermal and mechanical properties of the materials. Significant property differences observed with respect to analogous thermosets made by varying crosslink density using different extender/hardener ratios are the sensitivity of T-g to the crosslink density and enhanced fracture toughness and tensile yielding with reduced bulk density. (c) 2008 Elsevier Ltd. All rights reserved.