In supersonic tunnel tests with a two-dimensional backward-facing step model, span-wise perturbations were found in the reattachment region. Studies were made using the same and other similar models. In these later studies, the same three-dimensional perturbations were detected, into the full thickness of the boundary-layer, after reattachment, both in turbulent and laminar regions of the flow. Their amplitude was a maximum in the transition region. Using a model with a much improved accuracy of manufacture, notably at the leading-edge, it was found that the amplitude of the perturbations was greatly reduced, roughly in proportion to the size of the irregularities of manufacture of the leadingedge itself. It is concluded that the phenomenon is essentially one of instability in the twodimensional flow, the main triggering action arising from small irregularities in the leading edge.