Deflagration to detonation (DDT) experiments with stoichiometric hydrogen-methane-air mixtures at ambient pressure (1 atm) and temperature (293 K) were carried out in a 112 x 112 mm by 6 m long tube with various obstacle configurations. The hydrogen fraction in fuel mixtures ranged from 0 to 1. Two types square orifice plates with opening sides of 86.8 mm and 70.8 mm (blockage ratios of 0.4 and 0.6), spaced at 1, 2,3 and 4 times the tube height were employed. Flame signals were detected by photodiodes spaced evenly, from which the average velocity could determined. The detonation cell size was obtained using soot foil technique. CJ detonations can still propagate in less sensitive mixtures with larger detonation cell sizes. At the DDT limits, the ratio of the height of the orifice plate (d) and the detonation cell size (lambda) increases with the increase of the blockage ratio and the decrease of obstacle spacing. This indicate that in a square tube with square orifice plates, the detonation initiation process as well as the detonation propagation mechanism govern the DDT limits. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.