The spontaneous ignition of hydrogen at pressures between 3.5 and 7 MPa has been investigated. A free piston compressor was used to rapidly increase the temperature and pressure of a mixture of hydrogen, oxygen, and helium. Explosion occurred during the stroke, and was detected by a piezoelectric pressure transducer. The temperature at ignition is found to be independent of pressure, and is calculated to be approximately 1150 K. A chemical kinetic analysis was conducted, and the results are in good agreement with the experiments. The analysis indicates that the ignition is initiated by the breakdown of H2O2 to the highly reactive radical OH via the reaction H2O2 + M --> 20H + M. A new analytical expression for the ＇＇third limit＇＇ is derived from the reaction mechanism by introducing the concept of a critical OH concentration; it is shown to be in agreement with the experimental data.