When thiol self-assembled monolayers (SAMs) form on gold surfaces, it is widely believed that, upon adsorption, the thiol molecules dissociate via S-H bond scission. This mechanism is hard to verify since hydrogen is difficult to detect during this process. Hence, other reaction schemes such as nondissociative thiol adsorption have also been proposed. Here we present experimental evidence that clearly shows that hydrogen is released during dissociative thiol adsorption and interacts with the monolayer terminus. Vacuum vapor deposition was used to form SAMs of 4-nitrophenylthiol, 4'-nitro-1,1'-biphenyl-4-thiol, and bis-(4,4'-nitrophenyl)-disulfide on gold surfaces. X-ray photoelectron spectroscopy shows that the nitro groups of the thiol SAMs are partly reduced to amino groups, while those of the disulfide SAMs are not. The reduction is attributed to hydrogen released in the dissociation of S-H bonds during thiol adsorption. Possible pathways for the interaction of hydrogen with the nitro groups are discussed.