In the present work, self-ignition of high-pressure hydrogen that is released to air through a ruptured diaphragm is studied experimentally under various test conditions. Diaphragms with several thicknesses and scores are ruptured by high-pressure hydrogen, and the formation of a shock wave in the tube is experimentally observed. The behavior of the ruptured diaphragm and the subsequent self-ignition is photographed using a high-speed camera, while the self-emission intensity from a flame is measured using a photo multiplier. The experimental results show that the diaphragm rupturing condition is one of the main factors affecting the self-ignition phenomena. In addition, the self-ignition causes a ring-shaped flame that travels downstream, along a boundary between the hydrogen jet and the shock-heated air. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.