Directly quantifying and comparing the DNA damage inhibition of a range of sulfur compounds enables evaluation of the effects of structure on antioxidant activity. Of the sulfur compounds tested, GSSG, GSH, cystine, methionine, cysteine, and methyl-cysteine prevent DNA damage with CuI. The disulfide compounds GSSG and cystine have the lowest IC50 values, likely due to the presence of two sulfur atoms in these molecules. In addition, the thiol compounds GSH and cysteine inhibit approximately half the coppermediated DNA damage of their corresponding disulfides. Cystine