A numerical model based on the finite volume formulation to predict laminar flows hydrodynamics and mass transfer of aqueous solutions (570 < Sc < 3200) in the feed channel of spiral-wound and plate-and-frame systems is presented and experimentally validated. Particular attention is devoted to the physical modelling of the solute transport inside the membrane that yields parameters pertaining to solute-membrane interactions and are incorporated on the mass transfer boundary condition at the membrane surface. A correction factor, dependent on the solute mass concentration at the membrane surface, is proposed for the osmotic pressure phenomenological equation. The experimental cell is a slit (200 mm x 30 mm x 2 mm) that simulates the two-dimensional developing flow in the channels of spiral-wound modules. The predictions are validated against experimental data of apparent rejection coefficients and permeate fluxes, exhibiting an excellent agreement. A correlation for the concentration boundary layer thickness, a measure of the concentration polarisation, based on the predicted values of the solute concentration profiles, delta (omega)/h = 15.5 (l/h)(0.4) Re-0.4Sc-0.63 Re-p(-0.04)[1-186Sc(-1.0)Re(p)(-0.21)] is proposed in the operating condition ranges of 250 < Re < 1000, 0.02 < Re-p < 0.1 and 800 < Sc < 3200.