In the present paper the results of experiments on study of high-speed deflagrations in flat layer ofhydrogen-air mixtures unconfined from below are presented. The experiments were performed in twodifferent rectangular channels: small-scale with mixture volume up to 0.4 m3 and large-scale withvolume up to 5.5 m3. The main goal of the experiments was to examine the possibility of the layergeometries to maintain high-speed deflagration and detonation. With the aim to study a range ofcombustion regimes the experiments were performed varying degree of channel obstruction, hydrogenconcentration and thickness of the layer. Depending on the experimental conditions all majorcombustion regimes were observed: slow flame, fast ? ?choked? flame and steady-state detonation. Itwas found that minimum layer thickness in the range of 8 to 15 detonation cell widths is required forsustainable detonations.