The effects of low pressure on the laminar burning velocity and flame stability of H(2)/CO mixtures and equimolar H(2)/CO mixtures diluted with N(2) and CO(2) were studied experimentally and theoretically. Experiments were conducted at real sub-atmospheric conditions in three places located at high altitudes 500 m.a.s.l. (0.947 atm), 1550 m.a.s.l. (0.838 atm), and 2300 m.a.s.l. (0.767 atm). Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. The behavior of the laminar burning velocity at low pressures depends on the equivalence ratio considered; it decreases at lean and very rich equivalence ratios when pressure is increased. However, a contrary behavior was obtained at equivalence ratios corresponding to the highest values of the laminar burning velocity, where it increases as pressure increases. Numerical calculations were also conducted using a detailed reaction mechanism, and these do not reproduce the behavior obtained experimentally; a sensitivity analysis was carried out to examine the differences found. At lean equivalence ratios, flame instabilities were observed for all the syngas mixtures. The range of equivalence ratios where flames are stable increases at lower pressures. This behavior is due to the increase of the flame thickness, which considerably reduces the hydrodynamic instabilities in the flame front. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.