The comparison of the 20 Pong Su heroin samples with just over 200 authentic heroin samples from the 4 known heroin origins--Southeast Asia (n=59), Southwest Asia (n=37), South America (n=104), and Mexico (n=21)--found the Pong Su samples to be isotopically unique. The combination of the isotope and previous alkaloid data for Pong Su heroin samples clearly show it to be unlike any previously examined heroin. The origin of this heroin is unknown until authentic heroin samples of known origin produce similar data. Both continuous flow elemental analysis-isotope ratio mass spectrometry and gas chromatography-isotope ratio mass spectrometry techniques were used in the analysis. Heroin samples were also converted to morphine without apparent isotopic fractionation, using methanolic HC1 for gas chromatography-isotope ratio mass spectrometry. 3 tables, 4 figures, and 11 references