In this study, a series of experiments have been carried out to determine the detonation velocity deficits of 2H(2)-O-2 and 2H(2)-O-2-3Ar mixtures in narrow gaps. Those gaps include a 36 mm inner round tube and three annular channels gaps (w =2 mm, 4.5 mm and 7 mm, w is the gap of annular channel). Photodiodes and shock pins are employed to measure the time of arrival of the detonation wave. Smoked foils are used to record the cellular detonation structure. Experimental results indicate that for those detonations in 2H(2)-O-2 and 2H(2)-O-2-3Ar mixtures within the limits, the detonation propagates at a relatively steady velocity with small deficit, as the limits are approached by progressively reducing the initial pressure, the velocity gradually decreases and the failure of detonation occurs as the initial pressure below a critical value. In the 36 mm diameter round tube, the critical pressures for successful propagation detonation in 2H(2)-O2 and 2H(2)-O-2-3Ar mixtures are 13 kPa and 11 kPa respectively, and the maximum velocity deficits for above mixtures are 9% and 5%. By introducing a more accurate method that calculate the intermediate parameters (i.e., detonation cell size, reaction zone thickness) for Fay's model, a modified theoretical model to predict the detonation velocity deficits in different geometries is proposed. By comparing with the experimental results, it confirms the theoretical model is suitable for experimental data with reasonable accuracy in most cases. Due to the theoretical model does not consider the boundary condition, which is fairly prominent in very narrow gap (e.g., 2 mm annular channel), therefore, larger discrepancy between the experiment and theoretical prediction can be observed in the case of small gap. Both experimental and theoretical results indicate velocity deficit increases as detonation transmits from round tube to annular channel, which is due to the heat and momentum losses from wall reduces the propagation velocity of detonation. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.