This work is dedicated to an experimental study of overall mass transfer at the bottom of a rectangular cavity subjected to forced convective flow at its mouth. Mass transfer coefficients have been measured using an electrochemical method for different aspect ratios, H/h, between the depth of the cavity to the height of the upstream channel, range between 0.5 and 1.67. Reynolds number, based on the hydraulic diameter of the upstream duct, has been varied from 1 to 3000 and Schmidt number between 1010 and 8400. Overall mass transfer at the bottom edge of the cavity is found to increase with both Reynolds and Schmidt numbers and to decrease with the aspect ratio, H/h. General correlations taking into account these different parameters have been established in order to predict mass transfer at the bottom of the cavity. This work is the first part of a global investigation of a pervaporation membrane separation cell involving mass transfer in liquid phase at one side of the membrane surface, followed by a transfer of the species across the membrane and finally vaporization on the downstream side of the membrane occurs.