Methanesulfonic acid (MSA) is an interesting supporting electrolyte with many desirable properties such as high salt solubility, high conductivity, low corrosivity and toxicity. Various additives or complexing agents such as brighteners, antioxidants, and surfactants are required for deposition of Sn and Cu-Sn alloys. It has been shown that the simplest electrolyte for successful Cu-Sn deposition can contain just an antioxidant and a fluorosurfactant. In this work we have further examined the role of fluorosurfactant in shifting the electrochemical reduction potential for the more noble metal. The surface adsorption and desorption processes of the surfactant has been inferred through the use of an electrochemical quartz crystal nanobalance (EQCN). Cu and Sn have been deposited individually and simultaneously from MSA electrolytes. Mass changes at the quartz crystal have been compared against those calculated from charge consumption. These data show that in MSA electrolytes the adsorption and desorption of the fluorosurfactant to the surface is potential dependent and it suppresses Cu deposition but facilitates Cu-Sn alloy deposition. (C) 2015 The Electrochemical Society.