A method for determining a percentage of recycled polyethylene terephthalate [r-PET, hereinafter] in a material [material M, hereinafter] with regards to the total amount of r-PET and non-recycled polyethylene terephthalate comprised in said material M, said r-PET being derived from at least one polymeric material comprising at least one PET polymer and optionally IPA monomers and optionally TPA monomers wherein said at least one PET polymer is derived from recurring units derived from ethylene glycol and recurring units derived from terephthalic acid [TPA, hereinafter] and recurring units derived from isophthalic acid [IPA, hereinafter]; said method comprising the steps of: a) determining the molar ratio of IPA or the molar percentages of IPA of each reference material of a series of reference materials, each reference material comprising at least one PET polymer and optionally IPA monomers and optionally TPA monomers, wherein said at least one PET polymer is derived from recurring units derived from ethylene glycol and recurring units derived from TPA and recurring units derived from IPA; wherein the molar ratio of IPA of each reference material of said series of reference materials is the ratio of the molar quantity of the recurring units derived from IPA and when present the IPA monomers to the molar quantity of the recurring units derived from TPA and when present the TPA monomers as comprised in said each reference material of said series of reference materials; and wherein the molar percentage of IPA is the molar percentage of IPA recurring units and when present the IPA monomers as comprised in said each reference material of said series of reference materials; b) calculating the average of the molar ratios of IPA or of the molar percentages of IPA of the series of reference materials as determined in step a); c) providing a material M; d) determining the molar ratio of IPA or the molar percentage of IPA of said material M by analysing said material M using at least one analysis method selected from the group consisting of nuclear magnetic resonance spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, gas chromatography, high pressure liquid chromatography (HPLC), Near Infrared spectroscopy; wherein the molar ratio of IPA of said material M is the ratio of the molar quantity of the recurring units derived from IPA and when present the IPA monomers to the molar quantity of the recurring units derived from TPA and when present the TPA monomers as comprised in said material M; and wherein the molar percentage of IPA of said material M is the molar percentage of IPA recurring units and when present the IPA monomers as comprised in said material M; e) determining the percentage of r-PET in said material M by calculating the ratio R IPA wherein: R IPA = Y X * 100 and wherein X is the the average of the molar ratios of IPA or of the molar percentages of IPA of the series of reference materials as determined step b) and Y is the molar ratio of IPA or the molar percentage of IPA of said material M as determined in step d) and wherein X≥Y and X is different from 0. a) determining the molar ratio of IPA or the molar percentages of IPA of each reference material of a series of reference materials, each reference material comprising at least one PET polymer and optionally IPA monomers and optionally TPA monomers, wherein said at least one PET polymer is derived from recurring units derived from ethylene glycol and recurring units derived from TPA and recurring units derived from IPA; wherein the molar ratio of IPA of each reference material of said series of reference materials is the ratio of the molar quantity of the recurring units derived from IPA and when present the IPA monomers to the molar quantity of the recurring units derived from TPA and when present the TPA monomers as comprised in said each reference material of said series of reference materials; and wherein the molar percentage of IPA is the molar percentage of IPA recurring units and when present the IPA monomers as comprised in said each reference material of said series of reference materials; b) calculating the average of the molar ratios of IPA or of the molar percentages of IPA of the series of reference materials as determined in step a); c) providing a material M; d) determining the molar ratio of IPA or the molar percentage of IPA of said material M by analysing said material M using at least one analysis method selected from the group consisting of nuclear magnetic resonance spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, gas chromatography, high pressure liquid chromatography (HPLC), Near Infrared spectroscopy; wherein the molar ratio of IPA of said material M is the ratio of the molar quantity of the recurring units derived from IPA and when present the IPA monomers to the molar quantity of the recurring units derived from TPA and when present the TPA monomers as comprised in said material M; and wherein the molar percentage of IPA of said material M is the molar percentage of IPA recurring units and when present the IPA monomers as comprised in said material M; e) determining the percentage of r-PET in said material M by calculating the ratio R IPA wherein: R IPA = Y X * 100 and wherein X is the the average of the molar ratios of IPA or of the molar percentages of IPA of the series of reference materials as determined step b) and Y is the molar ratio of IPA or the molar percentage of IPA of said material M as determined in step d) and wherein X≥Y and X is different from 0.