The invention concerns a method of measuring the amount of a particular SERS-active taggant compound in a sample of a material which includes the steps of adding an internal standard containing an isotopically-altered version of said SERS-active taggant compound to the sample, contacting the sample/internal standard mixture with a SERS substrate then subjecting the mixture and SERS substrate to Raman spectroscopy. The concentration of SERS-active taggant compound in the sample is then calculated from the ratio of (i) the Raman spectroscopy detector response to the SERS-active taggant compound to (ii) the Raman spectroscopy detector response to the internal standard.