In this work, we investigate the ability to tune the quantity of surface amine functional groups in the interfacial region of epoxy-diamine composites using NEXAFS, a technique that is extremely sensitive to surface composition. Thereby, we employ a model surface (silicon wafer with the native oxide present) and, after deposition of an epoxy functionalized silane, we immersed the wafers in various diamines, followed by reaction with a diepoxy acting as a molecular probe. These results show that the number of available surface amines depends on the diamine chosen, wherein smaller molecular weight diamines provide more reaction sites. Subsequent experiments with mixtures of diamines undergoing competitive adsorption show that the amine quantity can be tailored by choice of the diamine mixture. Further experiments of diamine treated 3-(glycidoxypropyl) trimethoxysilane layers in a reacting epoxy/diamine showed that the surface reaction site density differences observed for adsorption experiments persisted in the reacting epoxy, implying that the surface reaction rate (and by extension, the surface amine concentration) dictate interfacial cross-link density up to the point of gelation.