Abstract: The method is applicable to protecting security documents of the type comprising a substrate bearing information which is to be protected from tampering, together with transparent protective layers of thermoplastic material heat sealed around said substrate. The method provides a set of pigment-retaining micro-holes in the substrate, said pigment being soluble in protective layer solvents. Any attempt at dissolving away the protective layers to gain access to the support for tampering with the information thereon, will also have the effect of making tell-tale marks on the support, as it absorbs the pigment is spread in the thickness of the support like ink in blotting paper.