Currently, the experimental data on the influence of vent position on the overpressure development during gas explosions in large length to diameter ratio (L/D) vessels are very sparse. Design guides such as NFPA 68 recommend the use of side-venting, but this is based on few experimental data and it was the objective of the present work to provide further information on vented explosions in tube configurations with side vents, Methane-air explosions (10% gas by volume) were undertaken in large L/D vessels and the influence of the side-venting position relative to the spark was investigated and compared with end-venting. The pressure development during side-venting was investigated and five stages of the explosion development were identified. The five pressure peaks were identified as follows: P-1 due to the initial elongated flame acceleration, P-2 due to the turbulence flame acceleration in the downstream gases ahead of the flame set in motion by the initial explosion gas expansion, P-3 due to the flame acceleration around the 90 degrees bend of the side vent, P-4 due to the external gas cloud explosion downstream of the vent, and P-5 due to the oscillatory combustion of trapped unburnt mixture in the vessel between the side vent and the far end of the tube, All these pressure peaks were not present for each vent position and the effect depended on the distance of the side vent from the ignition source, Very high flame speeds and overpressures were measured as the distance between the side vent position and the spark was increased, In the end-venting explosions the maximum overpressure was higher than the side-vented tests, except for the case where the side vent was placed at the middle of the vessel, which had a similar overpressure to end-venting, Induced gas velocities were also measured and associated turbulent parameters were calculated and were found to increase with an increase in the distance of the vent position from the spark. Copyright (C) 1996 Elsevier Science Ltd