In this work, the effect of environment and fatigue loading on the mechanical behaviour of a structural adhesive modified with thermally expandable particles (TEPs) was investigated. Tensile tests were performed to get the tensile properties of the TEPs-modified adhesive as a function of moisture. It was found that the presence of moisture affects the mechanical properties of TEPs-modified adhesive. Large increases in ductility and reductions in tensile strength and Young?s modulus were measured for the water-saturated specimens, but after drying the tensile properties are recovered. Further, fatigue tests were performed on single lap TEPs-modified adhesive joints (0, 10 and 20 wt% TEPs) and a decrease in fatigue life was found for TEPs-modified adhesive joints compared to unmodified joints. Finally, a scanning electron microscope (SEM) analysis was performed in order to examine the fracture surfaces of the tensile specimens tested. SEM analysis showed that the absorbed moisture changes the fracture mechanisms and the morphology of the specimens.