The minimum ignition energy (MIE) of hydrogen-air mixture is measured using capacitive spark discharge. First, the effect of humidity on MIE is studied. It is shown that the MIE is approximately constant when the relative humidity increases from 0% to 90% at room temperature. This indicates that humidity has no significant influence on MIE. Next, the effect of spark duration on MIE is studied. The spark duration is increased by connecting a series resistor to the spark circuit. It is shown that the MIE is approximately constant when the spark duration is varied from 5 ns to 1 ms. (c) 2006 Elsevier B.V. All rights reserved.