Experiments in an obstructed semi-confined vertical combustion channel with a height of 6 m (cross-section 0.4 x 0.4 m) inside a safety vessel of the hydrogen test center HYKA at the Karlsruhe Institute of Technology (KIT) are reported. In the work, homogeneous hydrogen-air-mixtures as well as mixtures with different well-defined H-2-concentration gradients were ignited either at the top or at the bottom end of the channel. The combustion characteristics were recorded using pressure sensors and sensors for the detection of the flame front that were distributed along the complete channel length. In the tests slow subsonic and fast sonic deflagrations as well as detonations were observed and the conditions for the flame acceleration (FA) to speed of sound and deflagration-to-detonation transition (DDT) are compared with the results of similar experiments performed earlier in a larger semi-confined horizontal channel. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.