The reductive desorption of self-assembled monolayers (SAMs) of alkanethiols and mercaptoalkanoic acids from gold has been examined in various alkaline solutions and supporting electrolytes using an electrochemical quartz crystal microbalance (EQCM). The desorption exhibits two voltammetric waves: a large dominant wave and another smaller wave at more negative potentials. The appearance of two waves is due to the heterogeneity in the substrate crystallinity and/or in the packing state of the thiol molecules. The desorption charge is calculated after consideration of the charge required to establish the double layer of the uncoated electrode. The mass change per mole of electrons (mpe) for desorption is determined by comparing the frequency change with the total charge passed upon desorption. The mpe increases linearly with the chain length and has a slope close to the mass of CH2, as is expected for a one-electron process. In the case of alkanethiol desorption, however, the mpe is much smaller than the molar mass of the desorbed alkanethiol. This difference is attributed to the simultaneous adsorption of cation species from solution, which is supported by the observed cation dependence of mpe. We estimated the molar mass of the desorbed mercaptoalkanoic acid by summing the mass of this cation species and the mpe for the mercaptoalkanoic acid SAM. From the value thus calculated, we conclude that the terminal carboxylate of the SAM is associated with the solvated cation. The solvation numbers are determined to be 1.6, 2.4-3.0, and 0.2 for Li + , K + , and Cs + , respectively.