Experiments were performed in the T4 shock tunnel to investigate the self ignition of hydrogen in a supersonic air stream. Hydrogen was injected into the flow over an inclined flat plate for oncoming Mach numbers of 7.9 to 8.0. The nozzle-supply enthalpy was kept between 3.1 and 3.4 MJ/kg and two different pressure levels were used in the tests. Measurements of surface pressures were used to infer the location of ignition but only small pressure increases were obtained when combustion occurred. Therefore multiple tests at nominally the same condition were used so that statistical methods could be used to identify the ignition lengths. The ignition lengths of the hydrogen air mixture directly behind a strong leading edge shock indicate that Pergament's method is able to predict the ignition length to within 35% for the observed autoignition over the range of conditions tested.