Burning velocities and flammability limits of gaseous mixtures of combustible gas (hydrogen and methane), oxidizer (oxygen and air), and diluent (nitrogen, argon, helium, carbon dioxide, steam, water aerosol formed by evaporation of superheated water) have been measured at elevated temperatures (up to 250Β°C) and pressures (up to 4.0 MPa). It was found that with increasing temperature, the flammability region is widened for all the mixtures studied. With increasing pressure, the flammable region for mixtures of hydrogen-oxygen-diluent is narrowed, except where steam or water aerosol is the diluent. A more complicated dependence of flammability limit on pressure is found for mixtures of hydrogen-oxygen steam. The influence of water aerosol (formed by the rapid evaporation of superheated water having an initial temperature of 150Β°C) on the flammability limits of methane in air was also investigated. It was found that the inerting action of such an aerosol is mainly due to the steam available within it; water droplets which are present in the mixture do not have a significant influence on the flammability limits. The dependences of burning velocity on temperature and pressure were investigated for stoichiometric hydrogen-air mixtures diluted by nitrogen and steam. It has been shown that, over the ranges of temperature and pressure considered, the temperature index of burning velocity is positive (that is, burning velocity increases with temperature), while the pressure index changes sign from positive to negative on dilution by inert agents. The observed differences in the influences of steam and nitrogen on burning velocity cannot be explained on the basis of a thermal mechanism alone and is mainly due to the more active role of water molecules in enhancing termolecular recombination reactions in the flame front. The influence of small quantities of methane on the burning velocity of hydrogen-air mixtures also was investigated.