This work presents the results of the large scale experiments with detonation propagating in hydrogen-air mixtures in partially confined geometries. The main aim of the work was to find the critical conditions for detonation propagation in semi-confined geometries with uniform and non-uniform hydrogen-air mixtures. The experimental facility consisted of rectangular 9 x 3 x 0.6 m channel open from the bottom, acceleration section and test section, safety vessel, gas injection and data acquisition system. Sooted plates technique was used as a witness of the detonation. The rectangular channel was placed in a 100 m(3) safety vessel. For uniform hydrogen-air mixtures experiments with four different channel heights h were performed: 8, 5, 3 and 2 cm. The critical hydrogen-air mixture height h(*) for which the detonation may propagate in a layer is close to the 3 cm which corresponds to approximately three detonation cell sizes. For non-uniform hydrogen-air mixture with hydrogen concentration slope equal approximately -1.1% H-2/cm the critical hydrogen concentration at the top of the layer is approximately equal 26% and the mean detonation layer height is close to the 8.5 cm corresponding to the hydrogen concentration at the bottom of the layer approximately equal 16-17%. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.