The original sulfur isotopic reference material was the Canyon Diablo Troilite (CDT), a meteorite recovered from Meteor Crater in Arizona. The Canyon Diablo Meteorite was chosen because it was thought to have a sulfur isotopic composition similar to the bulk Earth. However, the meteorite was later found to be isotopically heterogeneous with variations up to 0.4‰ (Beaudoin et al., 1994). This isotopic variability resulted in problems for the interlaboratory calibration of sulfur isotope measurements. A meeting of the IAEA in 1993 defined Vienna Canyon Diablo Troilite (VCDT) in an allusion to the earlier establishment of VSMOW. Like the original SMOW and VPDB, VCDT was never a physical material that could be measured but was still used as the definition of the sulfur isotopic scale. For the purposes of actually measuring S/S ratios, the IAEA defined the δS of IAEA-S-1 (originally called IAEA-NZ1) to be -0.30‰ relative to VCDT. These fairly recent changes to the sulfur isotope reference materials have greatly improved interlaboratory reproducibility (Coplen & Krouse, 1998).