Characteristics of extinction, acoustic response, and NO formation for diluted hydrogen-air diffusion flames at various pressures are investigated numerically by adopting a counterflow model. The results show that extinction can be classified into three regimes, where extinction strain rate increases with pressure at low pressures, decreases at moderate pressures, and increases again at high pressures. This behavior is caused hy the variation of chemical kinetics with pressure. Reduced mechanisms area derived based on respective kinetic schemes. At a specified strain rate, peak flame temperature increases with pressure, then decreases, and finally increases again at high pressure. The mole fractions of major radicals H, O, and OH decrease monotonically with pressure, whereas that of HO2 increases and then decreases at high pressures. Acoustic-pressure response based on the Rayleigh criterion shows distinct characteristics depending on pressure in each regime. At low pressures, pressure rise causes an increase in flame temperature and chain branching/recombination reaction rates. As a result, heat release increases, leading to an acoustic amplification. Similar phenomena are predicted at high pressures due to radical rebranching of H2O2. At moderate pressures, weak amplification is predicted since flame temperature decreases and recombination reaction dominates. The extended Zel＇dovich mechanism is dominant in NO formation at low pressures, and NO formation via the nitrogen dioxide intermediate is dominant at moderate pressures. Ai high pressures, radical rebranching reactions and relatively higher temperatures enhance the extended Zel＇dovich mechanism again. The emission index of NO shows a behavior similar to the peak-temperature variation with pressure. (C) 2000 by The Combustion Institute.