This study is concerned with the development of a tool to predict the long term behaviour of adhesively bonded steel joints aged in sea water. First, diffusion kinetics and the mechanisms governing the degradation of mechanical properties of an epoxy adhesive are described. These two sets of data were used in a Coupled finite element (FE) analysis to determine the stress state in double lap shear (DLS) specimens before and after aging. However, subsequent tests on DLS specimens indicated an adhesive and not cohesive failure mode, so this approach could not be used to predict failure in the present case without introducing an interfacial damage parameter. A second approach was therefore employed, in which modified Arcan samples were designed in order to identify directly how the failure envelope changed with aging. Tests were performed on these modified Arcan specimens under shear, tensile/shear and tensile loads before and after aging. The results from these tests have enabled a tension-shear failure envelope to be constructed, which may be used to predict failure in joints with more complex stress states. The application of a coupled diffusion-mechanical property approach is illustrated for the Arcan specimen loaded in tension, and its application to the prediction of failure behaviour after aging is discussed. (C) 2009 Elsevier Ltd. All rights reserved.