Overall mass-transfer coefficients in the laminar entrance regions of 30, 45, 60 ans 90 degrees isósceles triangular ducts were determinated by application of the naphthalene sublimation technique. I in accordance with the analogy between heat and mass- transfer, the experimental conditions corresponded to a heat-transfer situation carachterized by simultaneously developing velocity and temperature fields in a isósceles triangular duct with isothermal lateral walls and adiabatic base. Results were obtained for Reynolds numbers in the range between 100 and 1800, and length-to-hydraulic diameter ratio from 2 to 40, in order to cover a long range of the dimensionless streamwise coordinate values, mainly in the neighborhood of the entrance region. Analytical expressions for the mass-transfer coefficients were determined using the least squares method. In the present analysis the obtained results were employed in a theoretical study of a solar energy collector consisting of a vee-corrugated selective plate. Performace curves for many working conditions were presented.