Bis (2-ethylhexyl) phthalate (DEHP) is a commonly used plasticizer and is thought to have potential for disrupting human endocrine function and inducing tumorigenesis. DEHP has been shown to be ubiquitous in food, especially fatty foods, such as milk, cheese, and butter. Consequently, the U.S. Food and Drug Administration (US-FDA) and the European Food Safety Authority (EFSA), have keen interests in determining whether or not the presence of this phthalate in food is the result of contamination with synthetic DEHP, made from petroleum derived feed stocks, or is in-fact, the result of natural processes. Herein, the fraction of contemporary carbon (i.e., naturally produced) in DEHP was determined for each of seven 1.1 kg samples of unsalted market butter by accelerator mass spectrometry (AMS) at the Lawrence Livermore National Laboratory, after isolation and purification methods optimized to provide ~250 μg of the DEHP in butter containing ~0.7 mg/kg DEHP at a total carbon purity of 92.5 ± 1.2 % to 97.3 ± 1.0 % (n=3, 1σ as determined by gas chromatography-electron impact-mass spectrometry (GC- EIMS). Method blanks contributed 0.52 ± 0.19 μg to 1.08 ± 0.08 μg (n=3, 1σ) carbon as DEHP in individual butter isolates, and median exogenous carbon contamination, including (1) contributions from post-HPLC handling (1.8 ± 9.1 μg to 22.2 ± 9.7 μg), (2) method derived carbonates (31.2 ± 7.2 μg), and (3) matrix-inherent carbonates (median of 120 μg carbon), was 50%. After correcting for these interferences, the mean fraction of contemporary DEHP in butter was determined to be 0.0220 ± 0.0497 (n=5, 1σ). At the 95 % confidence interval, 97.8 ± 9.9 % of the DEHP in butter was petrogenic. To our knowledge, this is the first report of the fraction of contemporary DEHP isolated from market butter in the U.S.