A widespread method to avoid damage from incidental gas explosions is to use flame arresters. The new European Standard EN 12874 provides guidance for testing such flame arresters. Nevertheless, there are still unsolved problems as regards the effects of flame speed and transient explosion pressure, tests of unstable detonations and statistics of the deflagration-to-detonation transition (DDT). To better understand these phenomena, a transparent test set-up (pipe and model arrester) was built and the flame propagation was directly recorded using high-speed cameras. Flame velocity and flame pressure were determined in each phase of the explosion. Time-resolved observations of the flame propagation inside the flame arrester provided direct insight into the quenching potential and the flame transmission process. (C) 2003 Elsevier Ltd. All rights reserved.