Abstract-The free-interaction influence of thermal expansion process in the boundary layer gas flow is analysed in this paper, using the formalism of the Triple-Deck theory. The physical model considered herein is the forced convection of a gas flowing over a flat plate subject to a step change in the surface temperature, taking place at a certain distance from the leading edge. There is a fundamental parameter 7" w , defined as the ratio of the wall temperature to the free stream temperature. For values of T w close to one, the governing equations can be linearized and solved with the aid of the Fourier transform method. However, for values of this parameter not close to one, a numerical treatment is required to solve the governing equations. Using finite-difference methods, the numerical results for the pressure, skin friction, thickness displacement and Nusselt number are presented for different values of T". Finally, for a critical value of this temperature ratio, the boundary layer separates.