Subcritical debonding is of particular concern for microelectronic packaging, coating, and adhesive applications. Time-dependent subcritical debonding at polymer/substrate interfaces occurs at lower mechanical loads and strain energy release rates compared with those required for catastrophic interface fracture. In this work, the role of organosilane adhesion promoters, 3-aminopropyltriethoxysilane and glycidoxypropyltrimethoxysilane, in subcritical debonding of epoxy/glass interfaces under hygrothermal condition is investigated. The epoxy systems studied included a thermally cured bisphenol F-based epoxy resin and a bisphenol F-based epoxy resin cured with a UV active curing agent. Subcritical debonding results revealed that there are two regions, the threshold strain energy release rate (GTH) and the power law region, observed in subcritical debonding curves. Hygrothermal aging not only lowers the critical debonding driving energy required for debond extension (GC) but also lowers GTH, below which interfacial crack growth does not occur. Interestingly, applying silane adhesion promoters on glass surfaces increased GTH values and decreased debonding growth rate. Therefore, the subcritical debonding growth rate mechanism was found to be sensitive to interface chemistry. An attempt to correlate the results of critical and subcritical debonding was undertaken. It was found that as aging time increased, the role of subcritical debonding became less important compared with the critical debonding component. However, the presence of subcritical debonding at applied driving energies significantly below GC has important implications for the long-term reliability of interfaces.