A modified penetration theory for boundary layer transport has recently been developed by Trinh and Keey (1992a & b). The theory involves an assumption that the diffusion of a momentum, heat or mass element generated at the wall is carried away primarily by the convection parallel to the wall and a characteristic time, i.e. the diffusion time t(d), is introduced to link the unsteady state diffusion equation to the steady-state laminar boundary layer equation. This provides a powerful and yet simple transformation for the treatment of boundary layer transport problems. This study demonstrates that it is not necessary to have the above assumption before obtaining this characteristic time, and the convection term, used in the modified penetration theory, which is parallel to the wall, should be regarded as an effective convection term because the velocity, as it was derived, also quantitatively accounts for the convection in the direction normal to the wall.