An experimental study of flame and pressure development have been performed in a large scale 50 m3obstructed tube of diameter 2.5 m and length 10 m and with one end closed and one open to the atmosphere. Homogeneous clouds of variable concentrations of cither methane/air or propane/air mixtures have been accelerated in the tube by five regularly spaced obstacles which blocked off 30% of the tube cross-sectional area. Two types of ignition sources were used to initiate the explosions. The planar ignition mode had 19 weak match heads that covered the whole of the tube cross-section at the closed end of the tube. The point source ignition mode used two weak match heads located at the centre-line, also at the closed end of the tube. In addition to the conventional pressure and flame-arrival probes, some of the tests also included hot-wire probes for flow and turbulence measurements.