The adhesion of Diamond-Like Carbon (DLC) films on Poly(ethyleneterephthalate) (PET) sheet has been characterised by tensile testing, using simple equipment. Three theoretical analyses of the tensile test are described, and their validity has been studied by calculating in three different ways the adhesion of the same samples (DLC on PET). The theoretically predicted adhesion from the longitudinal strain required for coating debonding did not appear to be an appropriate indicator for coatings on polymers. An approach consisting of measuring either the minimum or the maximum crack spacing led to results which were more consistent with the literature. The adhesion of DLC films on PET has been increased by a factor of 1.8 when a 5 min argon plasma treatment is applied prior to deposition. Although quantitative results have to be considered with caution, this method is a simple and quick means of obtaining semi-quantitative results.