A series of tests were conducted to investigate how difficult lean hydrogen-air mixtures (4-10% hydrogen concentration) are to ignite. The commonly reported lean limit of combustion for hydrogen is 4.0%. The tests that were used to determine this assumed a quiescent hydrogen-air mixture and no upper limit on ignition energy or gap size for ignition. In this work, ignition tests were conducted with: (1) quiescent hydrogen-air mixtures and ignition arc lengths up to 3.2 mm (0.126 in), (2) mixtures in motion and arc lengths up to 1.5 mm (0.060 in), and (3) low-velocity mixtures with common electrical appliances as ignition sources. This work showed that the probability of igniting lean hydrogen-air mixtures is strongly affected by the ignition source, where successful ignition was a strong function of are length and a weaker function of gas velocity and ignition energy. The tests were conducted at 26-27 degrees C (78-80 degrees F). (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.