Separation of a supersonic boundary layer (or equivalently a hypersoni
c boundary layer in a region of weak global interaction) near a compre
ssion ramp is considered for moderate wall temperatures. For small ram
p angles, the flow in the vicinity of the ramp is described by the cla
ssical supersonic triple-deck structure governing a local viscous-invi
scid interaction. The boundary layer is known to exhibit recirculating
flow near the corner once the ramp angle exceeds a certain critical v
alue. Here it is shown that above a second and larger critical ramp an
gle, the boundary-layer flow develops an instability. The instability
appears to be associated with the occurrence of inflection points in t
he streamwise velocity profiles within the recirculation region and de
velops as a wave packet which remains stationary near the corner and g
rows in amplitude with time.