The adsorption of methanethiol and n-propanethiol on the Au(111) surface has been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-temperature scanning tunneling microscopy (LT-STM). Methanethiol desorbs molecularly from the chemisorbed monolayer at temperatures below 220 K in three overlapping desorption processes. No evidence for S-H or C-S bond cleavage has been found on the basis of three types of observations: (1) A mixture of chemisorbed CH3SD and CD3SH does not yield CD3SD, (2) no sulfur remains after desorption, and (3) no residual surface species remain when the adsorbed layer is heated to 300 K as measured by STM. On the other hand, when defects are introduced on the surface by ion bombardment, the desorption temperature of CH3SH is extended to 300 K and a small amount of dimethyl disulfide is observed to desorb at 410 K, indicating that S-H bond scission occurs on defect sites on Au(111) followed by dimerization of CH3S(a) species. Propanethiol also adsorbs nondissociatively on the Au(111) surface and desorbs from the surface below 250 K.