We investigated the fracture energy, Gc, and the threshold fracture energy, Gth, in water at 80 degrees C of interfaces between epoxy and layers of five silane coupling agents: (3-glycidoxypropyl)trimethoxysilane (GPS), (3-aminopropyl)trimethoxysilane (APS), [3-(phenylamino)propyl]trimethoxysilane(PAPS), (3-mercaptopropyl)trimethoxysilane (MPS), and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPS) on the native silicon oxide surface. While 10-nm-thick layers of these all had similar high Gc, APS had the best overall values of Gth, but all adhesion promoters, except for AEAPS, performed roughly similarly. The results from the XPS analysis of the silicon side of the fracture surfaces suggest that for all silane layers, the fracture occurs somewhere within that layer. When the silane coupling agents were added to the epoxy, the samples with the higher threshold fracture energies exhibited a thicker layer of the epoxy remaining on the native silicon oxide surface. Silane coupling agents that perform well as deposited layers do not necessarily perform well as additives to epoxies.