Laminar mass transfer on a plane surface (electrode) deformed by a square micro-obstacle is numerically studied by simulating the flow and mass transfer at the solid/liquid interface. The obstacle has the same order of magnitude than the diffusion layer thickness and thus a linear approximation for the velocity profile is used at the inlet flow section. It is shown that the velocity profile in the normal direction, is disturbed in no more than three times the obstacle＇s height. It means that such micro-obstacle can be considered ＇invisible＇ from the bulk how point of view. On the other hand, the recirculating vortex behind the obstacle is found to have an enhancement ef;ect on the mass transfer which is very important and strongly increases with the Reynolds and the Schmidt numbers,