We report an EQCM study of the oxidative deposition of butanethiol on gold from a KClO4 aqueous solution, The frequency change is found to be proportional to the mass of butanethiol oxidatively deposited on the gold substrate. Potential step experiments show that the rate of oxidative deposition is almost independent of the deposition potential between - 0.76 and - 0.40 V. This contrasts with the strong potential dependence of the rate of deposition of deprotonated butanethiolate. Mechanisms for the electrodeposition of thiol and thiolate are described and the effect of solvation on the rate of deposition is discussed.