Flame acceleration experiments were performed in hydrogen-air and hydrogen-air-steam mixtures in a 1.5 m-diameter, 5.7 m-high cylindrical vessel, fitted with repeated obstacles, to investigate the effects of obstacles on combustion behaviour. The obstacles were made of 16.5 cm-outer-diameter aluminum pipes. The study included a hydrogen concentration range of 10 to 20%, steam concentrations of up to 40%, obstacle spacing of 0.55 m to 1.1 m, and flow area blockages of 10% and 28%. The experiments were performed at an initial temperature of 128 degrees C and an initial pressure of 100 kPa. For all mixtures investigated the flame speed in the vessel increased rapidly, reached a peak value at approximately midheight, and then decreased. The flame speed increased as the hydrogen concentration increased. Addition of steam had a significant mitigating effect on combustion; the extent of mitigation, however, depended on steam concentration and obstacle configuration. Whereas steam was highly effective in reducing the flame speed in a one-column obstacle configuration (similar to 10% blockage), its effect. was small in the presence of three columns of obstacles (similar to 28% blockage). The flame speed increased as the blockage increased. An increase in the obstacle spacing caused a reduction in the flame speed. Over the range of investigations, the effect of initial pressure on flame speed was small.