Results of boundary-layer investigations on the leeward side of a 1: 12 scale model of the ELAC 1 configuration of a space transportation system are presented. The configuration has the shape of a thick delta wing with a rounded leading edge. The model length is 6 m; the experiments were carried out in the 8×6 m 2 low-speed German-Dutch-Windtunnel at Reynolds numbers up to Re=40·10 6 . In a first series of experiments mean velocity profiles were determined in the turbulent boundary layer on the leeward side of the model, with a single hot-wire probe in the plane of symmetry at four positions. Comparison calculations with a numerical solution of the boundary-layer equations showed good agreement up to angles of attack α=10° . In a second series of tests the laminar-turbulent transition of the flow and its separation near the rounded leading edge were investigated at three positions with multi-sensor hot-film arrays with 40, 56, and 96 elements. These measurements demonstrated that the flow near the rounded leading edge is markedly influenced by the nose radius.