Following earlier tests (Ref. 1), in the T.C.E .A. supersonic tunnel S-l (Ref. 2), on a two-dimensional backward-facing step model in which span-wise perturbations were found in the reattachment region, further studies have been made using the same and other similar models. In these later studies, the same three - dimensional perturbations were detected, into the full thickness of t he 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 ,,,as found that the amplitude of the perturbations was greatly reduced, roughly in proportion to the size of the irregularities of manufacture of the leading-edge itself. As it is unreasonable to expect these to be anything other than random in character, it is concluded that the phenomenon is essentially one of instability in the two-dimensional flow, the main triggering action arising, at least in the earlier tests, from small irregularities in the leading-edge.