Local mass-transfer coefficients in the laminar entrance region of a flat rectangular duct were determined by application of the naphthalene sublimation technique. In accordance with the analogy between heat and mass transfer, the experimental conditions corresponded to a heat-transfer situation characterized by simultaneously developing velocity and temperature fields in an isothermal-walled parallel plate channel. Results were obtained for Reynolds numbers in the range between 350 and 1500. By use of a dimensionless streamwise coordinate involving the Reynolds number as a scaling factor, it was found that the data for the various Reynolds numbers could be brought together. The experimental results were compared with analytical predictions and good agreement prevailed. Overall mass balances performed using independent measurement procedures were satisfied to within a few per cent.