The aim of this study was to estimate a procedure for the biobased carbon content of polypropylene (PP) resin isolated from composites containing additives or fillers on the basis of the carbon 14 concentration ratio, as measured by accelerated mass spectrometry (AMS). To reliably estimate the biobased carbon content of plastics, additives and fillers in the composites had to be removed because they often contain significant amounts of biobased carbon. To obtain specimens with purity suitable for estimation, an isolation procedure for PP from the composites was devised. The dissolution of the composites in 1,2,3,4-tetrahydronaphthalene at 150°C, followed by immediate centrifugation of the hot solution, yielded PP as semicrystalline precipitates by allowing the hot solution to cool during centrifugation. The recovery of the resin through the scooping off of the precipitates was typically 90%. This simple procedure provided a suitable specimen for the estimation of biobased carbon content by AMS on the basis of ASTM D 6866. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39978.