Detonations and Deflagration to Detonation Transition (DDT) represent a major hazard to process industry and fuel infrastructure. The main objective of this study was to experimentally investigate the location where DDT occurs behind one obstacle. A second objective was to identify the events leading up to DDT as well as the effects of inhomogeneous gas mixtures. High speed film and pressure records were the main diagnostic tools. Local explosions were observed at the walls and pressure waves oscillated between the walls, as seen on high speed film and recorded by pressure transducers. These transverse pressure waves were assumed to amplify and couple with the reaction zone of the deflagration, leading up to DDT. The onset of detonation was always observed at the walls for all homogeneous experiments. Inhomogeneous gas mixtures are very relevant for safety applications, as real gas leakages are typically inhomogeneous in composition. This paper shows that DDT occurred at the interface between reactants and air layer.