Viscoelastic structural adhesives show great promise in industrial and civil applications where a highly ductile connection between dissimilar components is required, at both low and high load frequencies. Suitable chemistries for these applications are silicones, polyurethanes and MS polymers. These adhesives show the required low modulus and high elongation at joint failure, while retaining satisfactory strength for long-term applications.However, the reliability of these adhesives has not been studied extensively. Partly because of the high initial variability on the mechanical performance of these joints. The aim of this research is to identify and correlate potential factors that have an effect on the performance of an MS polymer joint. Both experimental and numerical studies are used to attain these results.Both studies show that bond geometry has a marked effect on its mechanical performance. The experiments show that the assumption of a solid, uniform adhesive joint is invalid. However, if the size and location of defects in the bond line can be estimated, and an appropriate finite element model can be constructed, the joint performance can be estimated.