The validity of the scratch test as a method of assessing the adhesion of diamond-like carbon (DLC) on polymers has been studied. Sheets of 12 mum thick polyethylene terephthalate (PET) and 100 mum thick polypropylene (PP) were adhesively bonded to glass slides in order to perform the scratch tests. The critical load is defined as the load at which tensile cracks occur in the coating homogeneously throughout the scratch. It is shown that the type and the thickness of the adhesive used have an influence on the critical load value. However, the calculated values of the interfacial shear strength do not depend on the adhesive thickness, and qualitative results in agreement with the literature have thus been obtained. The influence of a nitrogen plasma pretreatment on the adhesion of DLC films on PET and PP has been determined by both scratch lest and tensile test techniques. The results follow the same trend and show that the scratch test technique is a good tool for semi-quantitative comparisons.