This document specifies a method of determining the biobased mass content in plastics products, based on the radiocarbon analysis and elemental analysis. This document is applicable to plastic products and plastic materials, polymer resins, monomers or additives, which are made from biobased and/or fossil-based constituents. This method can be applied provided that the plastic product contains the element carbon and that a statement giving its elemental composition and its biobased carbon content is available.