We have investigated the structure and properties of dithiothreitol (DTT) monolayers adsorbed on Au(111). Using scanning tunneling microscopy (STM), and Auger electron spectroscopy (AES), we have compared the properties of DTT monolayers with those of more conventional octanethiol layers. STM measurements indicate that much of the DTT layer is disordered; however, small ordered regions with local 2 3× 3 symmetry are observed. STM images of octanethiol films exhibit the characteristic close packed 3× 3 molecular layer with c(4×2) supersymmetry. AES indicates sulfur coverage for both the DTT and octanethiol films is similar. AES studies involving Ellman's reagent, a marker species, suggest a significant fraction of the DTT molecules in the adlayer bind to the gold via two Au-S bonds. On the basis of AES and STM measurements, we propose a structural model in which the majority of DTT molecules bind to the gold surface via two Au-S bonds. The distance between the two Au-S bonds is 5.00 Å, or the same 3a spacing observed between close packed alkanethiol molecules.