The present work investigated the effects of physical aging on interfacial adhesion and internal stresses using single-fibre model composites. First, the stress state of the interface and its evolution upon aging were studied for the microbond geometry. This stress analysis was used with experiments to determine an intrinsic interfacial shear strength. Second, this stress analysis, including aging effects, was used in a fracture mechanics or energy release rate analysis of the microbond test, which involves the energy necessary to propagate a crack instead of the minimum stress to initiate a crack. These methods were tested for both freshly cured and aged epoxy droplets on glass fibres, and were shown to yield similar results. It was found that neither the interfacial shear strength nor the interfacial toughness were changed by physical aging of the polymer matrix when the relaxation of internal stresses and shift of relaxation times of the matrix due to physical aging were accounted for. Thus, the intrinsic shear strength and toughness of the fibre–matrix interface were determined.