It is well-known that moisture attacks the interfacial bonds within an adhesive joint. A particularly interesting observation is that when the moisture level in certain systems exceeds a critical concentration, the bonded joint shows a dramatic loss of strength. A number of authors have investigated this phenomenon and concluded that the interface plays a crucial role but have not been able to explain why there is a critical concentration of moisture or what role is played by the properties of the bulk adhesive. Other researchers have shown that the concentration of water near the interface can be quite different than that in the bulk adhesive. If the interface is crucial to the observed effect, the local water content should be very important. To gain a clearer picture of what controls this phenomenon, we have combined a fracture mechanics approach to determine joint strength with neutron reflectivity which provides a detailed characterization of the moisture distribution near the interface. The work uses silicon dioxide substrates bonded to a series of polymers based on poly(n-alkyl methyl methacrylate). By substituting different alkyl groups, the molecular structure of the polymer can be systematically changed to vary the chemical and physical properties over a wide range. The results in this study suggest that the loss of adhesion is dependent on a combination of local water concentration near the interface, swelling stresses resulting from the water absorption, and water induced weakening of the interface bonds.