To investigate the electrostatic ignition of hydrogen-air mixtures, the density of OH radicals and the gas temperature are measured in an incipient spark-ignited hydrogen-air flame using laser-induced predissociation fluorescence (LIPF). The assessment of the electrostatic hazard of hydrogen is necessary for developing hydrogen-based energy systems in which hydrogen is used in fuel cells. The spark discharge occurs across a 2-mm gap with pulse duration approximately 10 ns. First, a hydrogen (50%)-air mixture is ignited by spark discharge with E = 1.35E(min), where E is the spark energy and E-min is the minimum ignition energy. In this mixture, OH density decreases after spark discharge. It is 3 x 1016 cm(-3) at t = 0 mu s and 4 x 10(15) cm(-3) at t = 100 mu s, where t is the postdischarge time. On the other hand, the gas temperature increases after spark discharge. It is 900 K at t = 30 mu s and 1400 K at t = 200 mu s. Next, a stoichiometric (hydrogen (30%)-air) mixture is ignited by spark discharge with E = 1.25E(min). In this mixture, OH density is approximately constant at 4 x 10(16) cm(-3) for 150 mu s after spark discharge, and the gas temperature increases from 1000 K (t = 0 mu s) to 1800 K (t = 150 mu s). (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.