Interfaces between 10nm thick 3-glycidoxypropyltrimethoxysilane (GPS) layers on silicon oxide-coated silicon wafers and amine cured epoxy were investigated. Before applying the epoxy the direct effect of the precuring temperature, Tpre, on the chemistry of the GPS layers was probed using Fourier-transformed infrared (FT-IR) spectroscopy. Swelling of the GPS layers when exposed to water or solvent vapor was measured by X-ray reflectometry. As expected, higher curing temperatures promote the siloxane formation process leading to near-completion of cross-linking around 250 degrees C. On the other hand, the epoxy groups of the GPS layer already disappear at a precuring temperature of 90 degrees C. Both the fracture energy (Gc) of dry Si/SiOx/GPS/epoxy interfaces at room temperature and the threshold (wet) fracture energy (Gth) of the interface in water at 80 degrees C were found to decrease progressively as the GPS layer was precured at increasing Tpre above 90 degrees C. At Tpre below 90 degrees C, Gc increased with decreasing Tpre while the Gth (at 80 degrees C) showed a peak for Tpre approximate to 80 degrees C. There is a strong correlation between swelling of the GPS layer in deuterated nitrobenzene vapor (reflecting the cross-link density of the GPS as well as the thermodynamic compatibility with polar organic molecules such as epoxy and amines) and Gth. On the other hand, the epoxy functionality of GPS appears to play only a minor role in creating an interface with epoxy that is resistant to water-assisted crack growth since a Gth that is close to the maximum is achieved at a Tpre (90 degrees C) at which the epoxy groups of the GPS have mostly disappeared. We conclude that for these GPS layers the interfaces with epoxy most resistant to water attack result under conditions where the mobile epoxy and amine species can interpenetrate a swellable precured GPS layer and that the reactive functionality of the GPS is relatively unimportant. The decrease in C and increase in Si on both sides of the fracture after curing at higher temperatures also supports this conclusion.