The self-ignition characteristics of H2-air-steam mixtures have been investigated in a heated shock tube for various temperatures, pressures and gas compositions behind the reflected shock wave. The measured ignition delay times show a strong dependence on the steam concentration and the temperature, while there is only a weak influence of the pressure for the conditions studied. The measured ignition delay times are consistent with the theoretical prediction only in the high temperature region. In the low temperature region the measured delay times are clearly shorter than those determined by the theoretical prediction. The pressure increase caused by ignition processes depends on the temperature behind the reflected shock wave. With increasing temperature the peak pressure first increases, then decreases. In terms of pressure signals the temperature can be divided into three regions: strong, transitional, and mild ignition regions. In the transitional region, a secondary explosion generates a high overpressure somewhere between the reflected shock wave and the end wall.