Reliable predictive modelling of the environmental degradation of adhesively bonded structures is required for a more widespread use of this joining technique. Recent durability modelling has coupled moisture diffusion and stress analysis, where the joint response is controlled by continuum degradation of the adhesive. However, the joint response is more commonly controlled by degradation of the interface. Current research extends existing durability modelling to include interfacial degradation and failure. Experimental studies have been undertaken to provide the moisture uptake parameters and moisture-dependent properties, both for the constitutive behaviour of a bulk epoxy and for the fracture energy of an epoxy-steel interface that has been exposed to various uptake levels of moisture. The mixed mode flexure (MMF) test was used to determine the interfacial strength. It was found that the interface fracture energy reduces with increasing interfacial moisture concentration. Interfacial rupture elements were developed to model the complete progression of damage within a joint from a single FE analysis. These rupture elements were formulated for mixed mode conditions and followed a separation law that used the fracture energy and the tripping strain as the controlling parameters. The role of these parameters was investigated, and it was shown that as long as there is a continuous process zone these elements respond well. This can be achieved as long as the tripping strain remains below a (mesh-dependent) critical value. Moisture-dependent fracture energies and tripping strains were then determined by calibration using the initial crack length data from the MMF specimens. These parameters were subsequently used to predict the response with increasing crack length, and excellent predictions were obtained.