Mass transfers in solution from or to an electrode surface occur at a micronic scale in many processes such as corrosion, electroless plating or electrocrystallization. Direct knowledge of local information is important for a better understanding of these processes. In this paper, we propose the use of an interferometric microscope to measure at a micronic scale concentration gradients which are directly related to a 3D mass transfer. After validation the technique with the Fe2+/Fe3+ electrochemical system, we have detected the local concentration gradient of the counter-ions exchanged between a ruthenium oxide film and its surrounding electrolyte with a spatial resolution of 5 micrometres. These preliminary results allow us to conclude that this method can be very fruitful for local mass transfer studies.