The classical Le ve ¼que solution of heat transfer induced by a small step change in the surface temperature in a shear ~ow "u linear in y# is revisited[ To obtain the shear ~ow we rescale the laminar channel ~ow of a perfect gas at a high Reynolds number in the Triple Deck scales and we investigate the retroaction of the temperature on the basic Poiseuille pro_le "near the wall the pro_le is a linear in y#[ This retroaction is achieved by two means\ _rst through the dependence of the viscosity and the density upon temperature and second through the gravity!induced transverse pressure gradient gauged by the inverse of the Froude number[ In the case of no transverse gradient a new self!similar solution is obtained showing that the skin friction at the lower wall is reduced by the heating while the one at the top wall is simultaneously increased[ In the general case with a Lower Deck based Froude number not in_nite\ the case of asymptotically small wall temperature variation allows a linearized solution which is solved with the Fourier transform method[ If the Froude number F is increased to in_nity we recover the preceding self!similar solution with small temperature variation[ If F is now decreased to zero we _nd that the leading term in 0:F of the solution shows that the skin friction at the lower wall is increased while that at the upper wall is decreased[ The conclusion is that the increase of temperature produces two opposite e}ects] _rst\ the expandability of the gas causes an upward displacement of the streamlines and a pressure decrease "the preceding self!similar solution is recovered with small tempterature variation#^second\ the buoyant e}ect produces the reverse e}ect of a downward displacement and a pressure increase which we believe may cause separation at the top wall in the non!linear case "skin friction at the lower wall is increased\ whereas it is decreased at the upper wall#[ These two e}ects qualitatively explain the ~ow computed with a full NavierÐStokes equation in an MOCVD reactor[ Þ 0888 Elsevier Science Ltd[ All rights reserved[