The effects of foreign gas mass addition of classical boundary layer magnitude on the compressible flow past a hot (cold) flat plate in a high‐speed uniform air flow are studied. In the case of a wall injection distributionm = C (2 x Re)−1/2, it is shown that there is a critical value ofCdepending upon the injected species properties, for which classical boundary layer theory fails. An interaction theory is developed which is uniformly valid in the limitRe→∞. The analysis is based on a three layer model consisting of inviscid rotational injectant regionO (Re−1/3)in thickness, a free viscous shear layerOO(Re−1/2), and the potential external flow. It is shown that the displacement effect of the thick injectant layer causes a correction to the external flow which results in a weak interaction favorable pressure gradient. The formulas for the pressure interaction and the wall shear indicate that the injection of a warm, low molecular weight gas has the most dramatic influence on the flow configuration. It is also shown that heat transfer to the wall is considerably reduced from the value predicted by classical boundary layer theory.