A theoretical study is made of periodic spanwise disturbances in nominally two-dimensional reattaching laminar and turbulent separated flows. A compressible small disturbance flow analysis of the local vortex instability mechanism involved is made emphasizing the three-dimensional heat-transfer effects including blowing or suction through the surface. It is shown that Reynolds analogy does not apply between the disturbance skin friction and heat transfer. The corresponding thermal response of the wall surface is also analyzed taking into account the spanwise heat conduction within the underlying surface materials; it is governed by the characteristic ratio of boundary layer heat transfer to the spanwise heat conduction. The theoretical predictions are in good agreement with experimental observations.