Experiments were performed to investigate the diffusion ignition process that occurs when hot inert gas (argon or nitrogen) is injected into the stoichiometric hydrogen-oxygen mixture at the test section. Detonation wave initiated by spark plug in the driver section in stoichiometric acetylene-oxygen mixture at P = 0.5 MPa and room temperature, propagates as incident shock wave in the driven section through inert gas after bursting the diaphragm separating the sections. At the end wall of driver section the inert gas is heated behind the reflected shock wave and then injected into the test section with the stoichiometric hydrogen-oxygen mixture through the hole 8 mm in diameter. An increase of the initial pressure of the combustible mixture in the test section from 0.2 to 0.6 MPa resulted in decrease of the minimum temperature of injected gas causing ignition from 1650 K to 850 K. At the same time the induction time for ignition process has increased from 190 to 320 s when hot argon was injected. For the injection of hot nitrogen an increase of the initial pressure of the combustible mixture from 0.2 to 0.4 MPa resulted in decrease of the minimum temperature of injected inert gas giving ignition from 1150 K to 850 K, and in increase of the induction time from 170 to 240 s. The results of experiments indicate that ignition occurs when the static enthalpy of injected mass of inert gas exceeds some critical value. The mechanism of ignition process was also studied by schlieren photography.