The hydrodynamic characteristics and the mass transfer were determined in the entrance region of an annulus with simultaneous development of velocity and concentration fields. A finite difference method was carried out to solve the problem by using the boundary layer assumptions. Two kinds of boundary conditions at the entrance were considered : uniform axial velocity and axial velocity at the exit of a porous media, which corresponds to experimental conditions. The velocity profiles at different axial locations were determined by laser Doppler velocimetry for different values of the Reynolds number. The wall shear stresses and the local mass transfer coefficients were obtained on microelectrodes by performing a polarographic method. The numerical predictions are in a good agreement with the experimental data.