This invention relates to the field of authentication of coating compositions such as varnishes, inks and paints, and it is particularly useful in the field of authentication of such coating compositions when applied to substrates like banknotes or other valuable documents. It is particularly directed to a marked coating composition. e.g. an ink, that is marked with a marker (taggant) such as to allow for its authentication, and a method for authenticating such a marked coating composition. The marking is achieved by covalently binding a taggant to a coating composition component. e.g. such used for security documents. The taggant, which is not extractable through usual chemical treatments like washing methods or the use of solvents, can be detected and identified upon thermally induced chemical fragmentation with a PY-GC-MS apparatus. The marked ink or the marked substrate is thus authenticated as belonging to a particularly marked ink, varnish or batch.