To avoid delamination which often occurs in microelectronic packaging as a result of widely varying thermal expansion coefficients, epoxy underfill is often loaded with particulate silica filler. This filler typically complicates the fracture behavior of the epoxy and surrounding layers. However, delamination of interfaces within the composite may contribute to toughening if the fracture path is adhesive between the epoxy and surrounding layers or otherwise removed from filler-epoxy delamination regions. In this work, the interface between silica filler and epoxy is altered with the use of silane coupling agents to elucidate the fracture and toughening mechanisms of filled epoxies. Two model epoxies are investigated, and coupling agents were chosen to encourage chemical reaction with each system. For the aliphatic epoxy system, coupling agents were identified which both augment and degrade the interface between filler and epoxy. In composite structures, it was observed that those coupling agents which degrade the filler-epoxy interface act as toughening agents. SEM indicates that most deformation is not at the filler-epoxy interface, but rather through the epoxy, very close to the filler particle surfaces. In contrast, no coupling agents were identified which act as toughening agents in the bisphenol-f system. Further, no variation in fracture toughness of composite structures was observed with the use of coupling agents, indicating that filler-epoxy delamination does not occur in this system. With few exceptions, SEM supports this interpretation.