Mass transport in a boundary layer with suction was studied for a parallel flow, laminar regime and high Peclet number. Mass transport mechanisms involved were analyzed and the respective mass transport fluxes were quantified by numerical methods. According to the magnitude of the convective fluxes, mass transport regimes were established. A simple, but accurate equation was deduced to identify the dominant convective flux and the transport regime. This identification only requires measurable variables combined in dimensionless groups. The accuracy of the equation was proved through the numerical solution of the governing flow and mass transport equations. The concentration field inside the mass boundary layer and the concentration polarization level at the permeable surface are intrinsically related with the dominant convective flux. A simple equation was deduced relating the concentration polarization level at the permeable surface and the parameter Omega(jz), which characterizes the transport regime. (C) 2003 Elsevier Ltd. All rights reserved.