Available experimental evidence on supercritical diffuser flow oscillations is reviewed, focusing on features that conflict with quasi-one-dimensional, linear acoustic theory. A model is proposed is which convective, transverse perturbations initiated by the shock and carried by the boundary layers are the dominant down-stream-moving disturbances in turn combine with upstream-moving acoustic waves to form the total oscillatory pattern. The boundary conditions at the two ends of the channel applicable to such flows are discussed. The reflection coefficients for pressure and velocity perturbations are related but not equal some estimates of their values are given. The model offers a qualitative explantation for most non-acoustic features of the oscillations and suggests a possible mechanism for inletcombustor coupling.