In the present work, the interaction of the fundamental anti-symmetric guided Lamb mode (Ao) with a structural discontinuity in a composite structure was studied through Finite Element numerical simulations and experiments. The structural component selected for this study was a T-joint section made from glass/epoxy material. This co-cured composite structure is made-up of an upper shell (skin) and a spar as the sub-components. It was observed that when Ao mode interacts with the junction (structural discontinuity) of these sub-components, a mode-converted So mode is generated. Experiments were conducted using air-coupled ultrasound to validate the numerical simulations. The back-propagating "Turning modes", which propagate from the thin region to the spar web and vice versa, were also numerically simulated and experimentally verified. © 2010 Elsevier B.V. All rights reserved.