Alkanethiolate monolayers on polycrystalline gold, formed by typical solution self-assembly conditions with immersion in dilute ethanolic solution of asymmetric dialkyl disulfides CH3(CH2)(n-1)SS(CH2)(15)CH3 (n = 4, 8, 12), were found to consist predominately of CH3(CH2)(15)S- moieties. This observation shows that large deviations from expected 1:1 compositions can occur. The data support an exchange mechanism involving dissociative chemisorption of the S-S moiety and a subsequent replacement of the shol ter chains by the longer ones via short chain disulfide or thiolate desorption and readsorption of mixed chain length disulfide. The efficiency of the substitution process increases with increasing length difference of the hydrocarbon chains in the respective disulfides.